I would say it has nothing to do with your assembly, but just the parts. The body probably needs more shaping to get the look that you want. I would assemble the legs and edit the body in the assembly so you can some faster feedback on the look.
I am sure Big Hero was not modeled in CAD, so the organic shaping behaves a lot differently, kind of. I don't know what level of Solidworks you have, but if you have the Sub-D ability that might be a good place to start as it will be easier to freehand a shape like this. Sub-D allows you to push and pull and make organic shapes much easier.
I would either flatten out the body on the bottom to reduce the V shape or bring out the tummy a bit to overlap the legs, hide the V shape and make it more round.
As with character modeling, I assume it is about making look right and not overly correct dimensions. This might also be easier to accomplish with surfaces and not solids.