Sure, show me a rollaboard with the same shape as a hard-sided spinner. That and the various capacity-limiting pockets make the overall available volume less than a spinner with recessed wheels. Compare the side view of a rollaboard and notice how much volume is lost because the shape is not maximizing the available external dimensions.

It is simply false that a rollaboard is necessarily more voluminous than a spinner. That is a common misconception perpetuated on the internet.