Well they are large enought to shuttle people around. Which is just what they're supposed to do. If you wan't to move large masses of people, the what is stopping you to stuff them in a crago container?
Isis and Elysium move handfuls of people or equipment at short distances. No need to be big and bulky...
Oh, and if you wan't to compare Seraphim and Azrael... Then why not compare modern day main battle tanks and APC's? Tanks are usually bigger since APC needs to be just big enought so it can carry a squad with equipment. No room for laughing (literally). Bigger also means slower, which also might not be desireable.
Yes, but the APC doesn't carry that many more people.
IIRC tanks have a crew of three or four, while APCs varies on the model. Some are just straight APCs with no guns, so they just need a driver. Some have guns, so they need a gunner. Then there are IFVs. Usually to make it worthwhile APCs should carry at least five troops. I thought China had a main battle tank that could transport infantry, but I forget.
Western modern MBTs generally have 4 crew members: Commander, Gunner, Loader and Driver. For tanks with autoloader (most tanks of Soviet lineage), the Loader is dropped for 3 crew members only. Most Western tank crews like having 4 guys, since the loader can help with the radio and general maintenance.
APCs like the M113 (APC varient) have only 2 crew members, Driver and Commander. The Commander also operates the MG mount at the top of the vehicle himself. For IFVs like the BMP series and Bradley, there are 3 crew members, Driver, Commander and Gunner.
In terms of carrying capacity, the M113 APC (not those with more fanciful turrets like the Israeli OWS or the CIS 40/50 turrets) can carry 11 fully equipped soldiers. IFVs generally carry far fewer soldiers, usually 6-7.
The only tank that I know of that can transport soldiers is the Israeli Merkava. Incidentally, the Israelis also converted some captured T55s into (heavily armoured) APCs.