Depends on the SpacingType parameter set for the Composite.
If you do not have minimum sizes set "Fill" will make everything fit by resizing everything equally if the Composite is not wide enough. If you think your composite should be wide enough but it still cuts of elements then check your "Spacing" parameter, it is likely too high.
EDIT: This may be different in game, see posts below: The defaul "spread" does not touch the sizes of the child elements but distributes them evenly across the width of the composite - or, which is probably what you are seeing, if the composite is not wide enough, cuts off the rightmost elements (without resizing them).
If you don't care about spreading and just want fixed distances use the "Contstant" SpacingType.
/end tutorial