Serious, response...
1) Size - the reason the distance squads are so big is mainly because the sprint / jump groups are so small. Those event groups are small because: a) Ivies are not in good training locations for those events; b) it is generally harder to get sprinters & jumpers accepted into the school (this is because of several more detailed reasons, but, in short, there are fewer sprinters & jumpers that meet the admission standard for the track team...and out of that limited pool, they often go to Princeton)
2) More on Size - the competitiveness of sprinting / jumping in the US college is system is absurdly high, and national-level competition stands to be dominated by only a handful of individuals...most of whom do not have the option to go Ivy (admissions standards, no scholarships, etc.). As an Ivy League school, it is far more realistic to become elite in cross country / relays
3) Academics - Staying enrolled & staying athletically eligible actually is not all that difficult at most Ivies, especially for the population that has been admitted. While some schools are certainly harder than others (Princeton with its thesis, junior paper requirements and grade deflation; Cornell for its general difficulty), some schools are quite easy (Brown, for instance, has no core requirements and gives the option to take any class pass-fail).
4) More on Academics - relative to other sports, track and cross country is among the most conducive to an athletes studies. Although the competitive season spans the entire school year, there are fewer periods of intense time pressure. Distance athletes will generally only have a maximum of two athletic related activities per day (2 runs, 1 lift & 1 run), and many athletes only train once per day. These training sessions are generally only 2-3 hours, often less. Ball sports often have periods of much more intense training. And, most importantly, because track & field training sessions can be completed individually, many coaches allow students some flexibility to reschedule training sessions around academic conflicts (e.g. a coach might oversee the same distance workout three times in one day - once with the main team, once with a small group that had a conflict, and once with an individual who couldn't make the other two sessions).