Well, I haven't heard of rmvb, but AVI is just a simple format.
You can compress these files using codecs which basically make the file smaller by taking short cuts, rather like MP3 files do.
Examples of compression are MPEG 2 (Used in DVD)
DIVX (A great format but needs a decompressor on the computer on the other end to run them, and not all computers have it)
and then of course, theres the simple Windows ones that come with players like Windows Media Player.
HOW IT WORKS:
The screen is split up into many squares of so many pixels, making blocks.
If a frame doesn't change for a frame, the compressor will say "right, that data there can stay exactly the same" (no more data needed),
But if only parts of the image are moving, for example if there was a white background with a moving circle, only the parts with the moving circle would be replaced every frame based on these blocks noticing changes, saving alot of space.
After the compressor has read the file and done all these changes, the computer your running it on needs a decompressor to decompress the data, and make it readable.
Without the right compressor, it just won't run, or if it did, your not likely to get much on the screen since each one uses a different formula.
Now you know the fundermentals, I'll try and find a compressor for you. Bare with me
CODECS AND CONVERTORS:
Heres the first one, DIVX I mentioned. Its actually better than DVD I think at high bitrates but offers excellent quality/vs space factors. So great for video:
Theres a trial, but I'm afraid you'd have to buy it, though the codec/decompressor itself is free, you couldn't convert your video to this without the convertor which isn't.
This is a free one which changes the formats of AVI to another one, but adds compression also. I haven't tried it, but it seems okay:
I'll keep hunting for you. But try these out and tell me what you think