BT had huge guns and exposed cockpits long before MWDA; exposed cockpits: Battlemaster, Timberwolf; huge guns: Marauder, Warhammer, Rifleman
Yeah, on mechs; it's an aesthetic, one that works in a game where you can only have 9 armour (normally) on any mech. No such weakness on a tank. I mean, unless you're playing with GI Joes; then yeah; huge canopy is the schtick.
And again; long apparent weapons, as on Marauder/Warhammer/Rifleman= realistic. Reference any book on tanks, artillery, warships. ect. You will see those kind of long, over-hanging gun barrels.
Now, let's look back at Gundam...
20 meters stinks of gundam and all those Japanimation type mecha to me. I like my mechs in the 10 meter range, though I agree there must be some variation by degrees.
So variation by degrees is fine, 20 meters is over the top. Okay, that's not a bad position to take; you allow some variance, but you set a limit. My limit is just higher than yours. But for me 20 meters is reserved for the biggest assault mechs; the atlas and pillager for instance. I'd peg the king crab at 7-8 meters by the way. I'd be fine with some light mechs around 5 meters tall, especially quads.
But lets revisit those guns.
Here's the pesky japanese aesthetic you don't like again again;
Now, I think this has it's place; done well it can work. Personally, I think the AC-20 on the Thunder, Hunchback and possibly the Centurion's Ac/10 are like the chest guns. Once in a while, I think the long, Gatling-type barrel makes sense. it can also be done poorly and often is and overdone, which it is now. Not much variety.
This, however is more what I see in MWDA designs;
. Note the huge bores, exposed ammo feeds, ect.