This issue has come up before - the problem is that it's far too hard to enforce for the staff. With a set number for each piece, it's easy to verify and remove offending material. To have to add the sizes together is a pain in the tail.
The current system is one that ended up working best for everyone, regardless of their connection. You can comfortably browse GBAtemp on a cellphone, and that's pretty cool. It doesn't take much to optimize your images/flash, and I for one am glad that we have these limits - brings more attention to the discussion, not the ludicrous sigs/avas some people choose to have. Some forums you can't enjoy the thread because every person has a ridiculous sig - and if the same person has more than one post worth of input to a thread..
I can't imagine you will find a lot of support for this idea within the rest of the staff either. The people who will support it the most are the ones with ludicrously sized sigs that have no place here anyway, or people that just haven't optimized their work enough. As many members around here have proved, you can do a LOT with 50Kb.
The 400x100 is a reasonable guideline, that really isn't enforced unless someone does something so far outside those restrictions that it's an eyesore, breaks the page, etc.
50Kb on a sig and 25Kb for an av keep the loading to a minimum and the enjoyability to a maximum. To add the limit together means a lot of 75Kb sigs with no avs, or as close to 75Kb sigs as possible with tiny avs. There aren't a lot of people here with 25kb avs..and really, what on Earth does a 75Kb sig do other than just clog the interweb?
