To add on to what Vince said (whassup!!!)...
Xingyi (as an example system) is low on content, but high on content... WTF?
When I was in Japan, learning to speak Japanese, my teacher told me that Japanese is "high context" while English is "low context."
In a similar vein, systems like Wing Chun, Xingyi, early Karate (all of which had/have only a very few forms) took what there was present in their system and milked it for its applications... Different angles of application, different planes of application, application of principles rather than techniques, etc. In this way, one or two forms could easily keep someone busy studying for a lifetime.
The "collecting" of forms that nearly all modern schools participate in is a hold over from when individual teachers would, having taught all they knew to their student, send them to learn from a friend or mutual acquaintence in order to learn something else to help them on the path. If Bob taught his student Sally katas 1, 2 and 3, and then sent her to learn from Sensei Tom down the road, who then taught her kata 4, Sally would, in all likelihood, teach her students katas 1 - 4. That is where the "kata collection" begins. It doesn't necessarily mean that Sally's training absolutely needed kata #4, but she kept it as it was useful to her. Her students learned 1 - 4, and maybe one or two of them went on to train elsewhere, learning katas 5 and 6. Et cetera.
One form could, reasonably, last an individual a decade and they'd never really have to look deeply into another one for that entire time. But find me someone who is going to do nothing by Naihanchi Shodan for 10 years... Not a common person, I'd bet.
