Infos coming in:
Turrican 3 was not downgraded to fit on one disk. There is a comment about this from Peter Thierolf, he did it merely for fun. The Mega Drive was just more powerful and could especially use many more colors in combination with paralax-scrolling.
But the soundtrack sounded much better on the Amiga, of course.
By the way, Super Turrican is even smaller:
Super Turrican: 512 KiB
Turrican 3: 880 KiB (or something near that)
Mega Turrican: 1024 KiB
Universal Soldier actually was Turrican 2. It was developed by the Code Monkeys (like Turrican 1 for Game Boy, PC Engine and Mega Drive) and when it was almost finished, somebody at Accolade (the publisher) decided to change it to Universal Soldier.