Such discrimination will in SOME cases probably be unlawful
An employer refusing to hire a person because of obesity might be in violation of the California CFEHA, or the Federal ADA, or both.
The California Fair Employment and Housing Act (CFEHA) will in some cases prohibit such discrimination.
Specifically, CA Code section 12940 says:
It is an unlawful employment practice, unless based upon a bona fide occupational qualification, or, except where based upon applicable security regulations established by the United States or the State of California:
(a) For an employer, because of the race, religious creed, color, national origin, ancestry, physical disability, mental disability, medical condition, genetic information, marital status, sex, gender, gender identity, gender expression, age, sexual orientation, or veteran or military status of any person, to refuse to hire or employ the person or to refuse to select the person for a training program leading to employment, or to bar or to discharge the person from employment or from a training program leading to employment, or to discriminate against the person in compensation or in terms, conditions, or privileges of employment. (Emphasis added)
CA Code section 12926 defines "physical disability":
(m) “Physical disability” includes, but is not limited to, all of the following:
(1) Having any physiological disease, disorder, condition, cosmetic disfigurement, or anatomical loss that does both of the following:
(A) Affects one or more of the following body systems: neurological, immunological, musculoskeletal, special sense organs, respiratory, including speech organs, cardiovascular, reproductive, digestive, genitourinary, hemic and lymphatic, skin, and endocrine.
(B) Limits a major life activity. For purposes of this section:
(i) “Limits” shall be determined without regard to mitigating measures such as medications, assistive devices, prosthetics, or reasonable accommodations, unless the mitigating measure itself limits a major life activity.
(ii) A physiological disease, disorder, condition, cosmetic disfigurement, or anatomical loss limits a major life activity if it makes the achievement of the major life activity difficult.
(iii) “Major life activities” shall be broadly construed and includes physical, mental, and social activities and working.
(2) Any other health impairment not described in paragraph (1) that requires special education or related services.
(3) Having a record or history of a disease, disorder, condition, cosmetic disfigurement, anatomical loss, or health impairment described in paragraph (1) or (2), which is known to the employer or other entity covered by this part.
(4) Being regarded or treated by the employer or other entity covered by this part as having, or having had, any physical condition that makes achievement of a major life activity difficult.
(5) Being regarded or treated by the employer or other entity covered by this part as having, or having had, a disease, disorder, condition, cosmetic disfigurement, anatomical loss, or health impairment that has no present disabling effect but may become a physical disability as described in paragraph (1) or (2).
(6) “Physical disability” does not include sexual behavior disorders, compulsive gambling, kleptomania, pyromania, or psychoactive substance use disorders resulting from the current unlawful use of controlled substances or other drugs.
Serious cases of obesity would probably be included in 12926 (m) (1). Less serious cases might be included in 12926 (m) (4) or 12926 (m) (5).
In addition, the US Federal Americans with Disabilities act (ADA) (42 U.S.C. § 12101) provides in section 12112 that:
(a) General rule
No covered entity shall discriminate against a qualified individual on the basis of disability in regard to job application procedures, the hiring, advancement, or discharge of employees, employee compensation, job training, and other terms, conditions, and privileges of employment.
As used in subsection (a) of this section, the term "discriminate against a qualified individual on the basis of disability" includes
(1) limiting, segregating, or classifying a job applicant or employee in a way that adversely affects the opportunities or status of such applicant or employee because of the disability of such applicant or employee;
(2) participating in a contractual or other arrangement or relationship that has the effect of subjecting a covered entity's qualified applicant or employee with a disability to the discrimination prohibited by this subchapter (such relationship includes a relationship with an employment or referral agency, labor union, an organization providing fringe benefits to an employee of the covered entity, or an organization providing training and apprenticeship programs);
(3) utilizing standards, criteria, or methods of administration
(A) that have the effect of discrimination on the basis of disability;
(B) that perpetuates the discrimination of others who are subject to common administrative control;
The ADA defines disability in section 12102:
Sec. 12102. Definition of disability
As used in this chapter:
The term "disability" means, with respect to an individual
(A) a physical or mental impairment that substantially limits one or more major life activities of such individual;
(B) a record of such an impairment; or
(C) being regarded as having such an impairment (as described in paragraph (3)).
(2) Major Life Activities
(A) In general
For purposes of paragraph (1), major life activities include, but are not limited to, caring for oneself, performing manual tasks, seeing, hearing, eating, sleeping, walking, standing, lifting, bending, speaking, breathing, learning, reading, concentrating, thinking, communicating, and working.
(B) Major bodily functions
For purposes of paragraph (1), a major life activity also includes the operation of a major bodily function, including but not limited to, functions of the immune system, normal cell growth, digestive, bowel, bladder, neurological, brain, respiratory, circulatory, endocrine, and reproductive functions.
Whether a particular obese individual will be regarded as having a disability under either the CFEHA or the ADA is a matter of individual determination. Not all obese people will be considered to have a disability as defined by either act (and note that the definitions are quite similar). If a person is so regarded, then to refuse to hire that person because of that disability is unlawful. Note that if a person is unable to perform the essential duties of a job, even with a reasonable accommodation, an employer is free not to hire such a person. There are other limitations and exceptions in each act. To determine if a particular person is protected in regard to a particular job would require an employment lawyer or other employment professional with access to the specific facts of the case.