I expect part of the holistic process does involve filtering students based on intended major. It would just be one of the parameters of admission hidden behind the word holistic. Yes, Harvard increased their size but there is no reason to make the assumption that the size was increased relative to the number of desirable applications to the area. While Harvard has tons of money, they would still need to hire faculty and make sure they have the other resources available - tutors, labs etc in a very short window of time to meet demand. I expect there would be a mismatch to some extent. On top of this, I don’t know if the school would want to increase the size of the incoming class to that extent when it knows that CS is one of those boom and bust fields. Right now CS may be popular but after the last boom before 2001 look what happened? Far less went into CS. Would they have wanted to hire a large number of faculty to accommodate the size of a boom class only to likely be under-enrolled during the bust cycles?
There are other problems with over-enrollment besides this issue. There are only so many facilities available and I expect this would affect STEM areas which require lab spaces more than other fields such as humanities or a STEM area such as Math. Often, when a field starts growing in popularity as Engineering has done, in order to accommodate students, new facilities need to be constructed.
Also schools do not want to flood the job market with graduates from one field who then can’t find a job so I would expect they grow classes more gradually than follow demand trends. Anyway, I see no reason to assume that the number of applicants to slots fills in naturally during some holistic process, especially as the data we have from Oxford or from state schools here suggest that certain majors are just harder to get into than others.