Submiting a two page paper to a math journal
I am no expert in number theory, so I can't really help you with the last question (if you should submit your paper to another journal). However, it sounds like the editor stated the reason for rejection pretty clear: In his impression the results are not substantial enough. This is a valid (and frequent) reason for rejection. So in this respect you are in good company. If the editor did not state explicitly that he finds the paper "too short" than there is no reason to believe that this was the reason for rejection.

To get good feedback on how to proceed with your paper and your research you should ask your advisor (or find one).