Photoshop's compression is LZW, like zip, I'd guess, also like TIF, and I'm sure it is non-lossy. RLE (run length encoded) doesn't do very good on 24 bit color, often the files get bigger, LZW typically reduces 20 - 30% in file size, as you said. I think Photoshop started compressing the files in version 4, or it might have been 3. I know version 4 had a preference for saving files in 2.5 format, I'd guess 2.5 was uncompressed, 3.0 + compressed, but 4+ should have no problem reading either type of Poser save.