Buy a used PS2 copy on eBay for $10-$15. Rip your own ISO using a tool like ImgBurn. Then compress it yourself with 7-Zip (Ultra compression setting). This gives you a clean, legal, highly compressed file without shady websites.
Unlike the PC/PS3/Xbox 360 versions, the PS2 and Wii versions were built on a different engine with smaller level layouts and streamlined puzzles. tomb raider underworld ps2 iso highly compressed