I had only tried the full version before, but just tried the 11mb version and it doesn't look any different in terms of graphics, it does suggests the difference is no intro and no music, so yes it's very impressive getting the full game down to 11mb.
Reading the guys website suggests he has done a full rewrite that looked like a PITA based off a shambolic codebase.
I only played the first one on the original PSX, it was one of the first games that came out for it.