Id really like to see a picture of two large hides capable of holding an adult female BCI and a large water dish on top of it in a 4x2....given that the snake itself occupies almost 1/2 of the floorspace of the enclosure by itself. If you count crawling on top of the hides as "room to move around" then I can see where you're coming from. Two large hides (the size of an inverted litter box) and a large dish gives you about 12"x12" of remaining floorspace. RBI jumbos as 20"x14"x6"....use two of them in a 4x2 animal plastics and youve got less than 2'x2' of floorspace.....then subtact your large water dish......
Muscle tone in snakes (like any other animal) is gained through activity...climbing for example. I fail to see how you would be getting muscle tone by using a small hide. There's an old trick where people use a 6ft step ladder for building muscle tone in large boas actually.