There are different ideas about how to best choose memory limits for your user groups and your program groups. The main idea is to stop runaway users and/or programs, so leaving some extra room isn't so bad if you're still stopping runaways.
A limit of 1GB for a certain program group might be far less confining than a 1GB limit for a user group, where no individual user's collected activity would be allowed to exceed 1GB. You would want to allow individual users more latitude than programs.
A user group memory limit on a box with 16GB of memory could reasonably be 4GB on the high end. This would prevent unnecessarily confining your users' activity while preventing the worst runaways.
Of course, the best settings will always depend on your particular users' habits and your own hardware.