None of the Ivies admit by major, other than Cornell and UPenn which admit by colleges. Some colleges with engineering schools may ask for an engineering supplement. However, each college does try to shape its class by having a good balance of admitted students with varied academic interests.
I will relay a personal anecdote. I have been a long time interviewer for Yale. Probably the worst assessment I have given on an interview involved a candidate that expressed an interest in a relatively obscure major, but one in which I have an interest and have done a fair amount of reading. I was looking forward to our conversation. Unfortunately the interviewee had barely superficial knowledge in the topic. During the course of the rest of the interviewer, it became obvious to me that the interviewee was really a pre-med type and must have gotten bad advice that the applicant should try a seemingly less competitive door. AO’s with access to all of the applicant’s file will easily spot that type of gaming.
All the competitive applicants to the highly selective schools you are considering will be incredibly strong. Your best bet is to present your strengths and not worry about anyone else. Your application is really a sales document. Ones that provide a clear, consistent and compelling picture of why you will be a contributor to the school will be the successful ones.