*Forcecage* only cares about the space (threat range, actual size, or otherwise) a creature takes up in its environment.

This is often measured on a grid. Medium creatures take up a 5 X 5 'space' even though they are not 5ft wide.  This represents a *Size* that is 'Medium'. Large creatures take up a 10 X 10 'space' . So on an so forth.

The Force cage can only have one of 2 dimensions: A 20 X 20 Cage with 1/2 inch spaced bars a 1/2 inch thick, or a 10 X 10 cube completely sealed and solid.

The spell cares about if a creature's size threshold (the dimensions it takes up as a result) fits within the chosen form. Any creature larger than 20 X 20 if cage is chosen or 10 X 10 if cube is chosen does not fit and is not 'completely inside the area'.

I am not at my computer and forget the relevant size categories but any creature 21 X 21 and larger would not fit.