That might be a bigger issue if Stormtroopers were riding around in WW2 tanks versus their own hover tank. There's no reason why the imperial tanks couldn't have larger hatches.
Except like, they really don't have that.
They're pretty typical looking of WW2 armor hatches in their design in many respects... The AT-ST's as well, but also this new tank.
My friend I was talking about this with isn't a SW fan as much a history guy, but we felt they're very similarly styled to a WW2 German tank destroyer/mobile gun... At least the profile from the front is similar. Very low, very cramped (looking anyway, without any inside details). The Commander's cupola is the most spacious looking hatch that can be seen (as most commander's cupolas are larger), and very WW2 German-ish looking itself actually, but still tight and made more-so by someone wearing armor obviously. I wouldn't be surprised at them pulling from WW2 weapons to design it since Star Wars has a history of that anyway in its look for the Empire.
The driver's hatch, which only the driver's head is visible, is insanely tight and I'd dare say almost impossible for him to remove himself if he's wearing armor besides a helmet and perhaps a tight/thin or flexible body piece. You only see his bucket though, so it's possible he's in a jumpsuit and no body armor.
Hatches are small for structural purposes more than anything, or they'd have larger hatches on actual tanks. Modern armor the shapes have changed, but they're still fairly tight squeezes to get into/out of, and are still largely rounded. Larger hatches mean larger weak points that can be targeted though. The top of the tank is generally weak (in real life anyway), so you try to minimize that... The size of the hatches on Star Wars vehicles like this (or the AT-ST) make you think that's possibly true in Star Wars too, and they want to minimize weak points on it.
This tank's real purpose is seemingly mixed... But the hatches are are tight for someone in armor to deal with it, whether they want inside it, out of it, or whatever. In an emergency it's REALLY bad if you're wearing armor and in something you have to get out of pretty fast.
I can't buy into these guys then being soldiers who have a need to exit fast, fight, get back in quickly again, fight some more... It's just not looking designed for that. Hell it's tough to say if there are even more than 3 or 4 people able to fit in that, actually.
Real armor DOES have the purpose of throwing men into combat though, actually... Tanks don't specifically, but Armored Fighting Vehicles like the Bradley, that's half their purpose generally (or acting as an ambulance, or command vehicle, whatever). They're formidably armed too, but are also a transport. This SW tank though, doesn't have a compartment in the rear for troops from anything I've seen, and like the pictures show the hatches are not large enough for more than a man at a time to deploy. It wouldn't be practical then even if it could.
Add in that hatches are often focal targets for snipers in combat zones (logical), and having the armor on making it slower to get in/out, strikes me as a position nobody would want to be in.
I still absolutely love seeing, for the first time, an Imperial vehicle other than a walker in action... It's just cool that they're looking at the broader scope. I wish we'd gotten that Rebel armor piece from Hoth made... It's obscure but not a large vehicle it seems. Be neat to have that in the collection. Rogue One is a visual treasure trove when they put out trailers... I love seeing new OT era "stuff". Especially now that isn't in some obscure RPG book's pages or video games.
EDIT: Yikes that pic was definitely not shrunk down by the site that posted it.