While updating the subview's frame (it needn't be a subview, by the way, but the coordinate logic below assumes so), check that the constraint
CGRect intersection = CGRectIntersection(subview.frame, superview.bounds);
BOOL fullyContained = CGSizeEqualToSize(intersection.size, subview.size);
More compactly, as @ZevEisenberg helpfully suggests:
BOOL fullyContained = CGRectContainsRect(subview.frame, superview.bounds);
If both views belong to the same superview, which is fine so long as the visually contained view is stacked on top (appears after the larger image view in the superview's subview array), then even easier:
BOOL fullyContained = CGRectContainsRect(subview.frame, superview.frame);