It is not the figure proportions, I agree with you that making them visually compatible with other popular brands is important.
It is the apparent gap between the figure and the seat, visible in my edited post above.
Tyroflyer is possibly right that by judicious filing they might sit more "heavily" on the seat cushion, but it will affect the angle of the legs and other parts of the figure. Having to cut the feet off a figure designed for a vehicle to make it fit may send the wrong signals to prospective customers. YMMV.