The two big problems I seem to have on big maps is:
1. I colonise many planets quicky, and when I first get them, they have like no people on them, so the maintinance of the itintial colony outways the taxes pretty heftily. Solution to this is I always have a very high population growth to get my taxes up to par (with a little help from my freind the recruitment centre too, on every single planet)
2. The tourism money, yes, I'm pretty sure it is linked to your influence in the galaxy (may to related to the area you control, but im pretty sure you get more from having 25% of a massive map, than 25% of a small map), not sure quite what you can do about this, but influence resources may be very helpful.
Oh and one other little side note, War profitiring is a lot more useful on large maps than small ones, if you know what I mean.