You have to remember. Guns like these were never designed for fast loading. They are designed to be riot type guns... this one in particular was designed for prison environments. They are intended to start out loaded with as many rounds as possible, that is their strong point. They were never designed to be quickly re-loaded. A shotgun that holds 15 rounds is going to have some trade offs... fast loading and more girth are the trade offs. The KSG is a WAY smaller gun... but after handling one, I actually like how the UTAS feels better. Its surprisingly light and easy to handle.
A semi auto of any kind in canada is what it is. There are many semi auto 12 gauge guns out there. Benelli M4 seems to be exactly what your describing. The new one from Uzkon is mag fed but is a bull pup. There is a mag fed bull pup version of the T97 as well (can't remember its name). So slim, short, semi auto 12 gauge guns exist. But being limited to 5 rounds is what really hurts them.