Phuket is the largest island in Thailand, Ko Chang is the second largest. It may not feel like a smaller Island because its so big. Ko Tao is one of the smaller islands, I dont know about any boxing camps there. Islands will always be more expensive than land, I lived on one for a year and had to bring everything in by ferry, it was expensive.
Ko Tao is more famous for diving, Ko Samui has boxing camps and feels like a island if that's what your looking for. Basically you have Bangkok/Pattaya, Phuket, and Chaing Mai, as far as boxing destinations. Outside of that you have a bunch of smaller camps scattered throughout Thailand. Personally I prefer the Isaan area, great prices, in the country, not as commercial as far as tourism. If you do your homework you can train anywhere in Thailand, some places just offer more or less comforts and focus, Muay Thai is everywhere.