Fluent, in short, the condom provides flexible counterpressure right at the skin for 100% of the shaft and head. The idea is that it restrains superficial expansion and encourages deep expansion. Since you’re not packing the tube, vacuum force is still being applied to most or all of your shaft and glans, so whole penis expansion is possible.
By contrast, if you were just to use a smaller cylinder, you would not have that restraint for 100% of the shaft and head unless you were packing the cylinder. Where you were packing the cylinder, there is no longer any external vacuum being applied to allow for further expansion there. If you were completely packing the cylinder, you would have no more room for expansion anywhere you were packing, and any force being applied via vacuum would be focused on the glans, increasing the chance of water blisters.
So, using a smaller cylinder does not accomplish the same thing as pumping with a condom.