I've been doing everything I can to reduce this problem in my mod (Dominion Wars). First of all, cutting down the texture size is a good idea, 256x256 is the minimum, although with some Fed ships, you may want to keep some textures at 512x512. All illumination files should be no more than 256x256. Then, reduce the number of colours down from True Colour, to 8 bit, or 256 colours. You'd think that the reduction in number of colours would reduce the quality of the texture, but in 99% of cases, the difference is minimal. And a 8 bit bitmap is only 65K, whereas a true colour is 192K (at 256x256). So as you can see, it's a big saving. All illumination & damage maps should be 8 bit.
Now all that matters is the complexity & size of the model itself.