The Telgorn Gamma class assault transport.
This one in non-standard Imperial colour scheme what was used in, IIRC, a computering game or two, of some kind.
Yes, it is intentionally shorter than the canon version. The reason, well, the reason would take quite a long time to explain fully, but is related to the standard squad size for imperial troops being eight, thus making a platoon of thirty two rather than forty, and the nature of operating units specifically relating to C3 issues.
If the Gammy were retrofitted for the spacetrooper, then forty slots could make sense. IMHO, though, the level of specific engineering makes it very likely this version was the first version and thus would have been to house a platoon, not a platoon plus an extraneous attached squad.
Also, I rather think the original illustration was for something else, as the size of the main canopy, the VAST canopy, the GIGANTOMONIC, the TITANOIDOBULBAJINKINGLY GODZOOKEROUS canopy, was probably intended to indicate a ship about the size of a Winnebago. My guess, and guess it be, no more no less, is that the Gamma was originally something like a arsenal ship, but MOST likely, was intended to house normal stormtroopers, and was legend-adjusted so the doors would fit the zero-g troopers, and they basically ignored the now CARCHAXPANSATORIALICIOUS front end because, well, that's just the way the cookiee crumbled at WEG.