Only the newer BoPs are larger, probably for resons like the need to put those weapons, stronger hull, shields and a power source to keep it all running somewhere.
That's why the models steadily grow in size from the B'rel up.
It makes sense otherwise it's like placing a Main Battle Tank's gun turret onto a jeep.
Even thugh there is no recoil in space, the weaponry must be moved and powered somehow.
Also while the Hegh'ta has a large wingspan and long hull this is not entirely the defining factor for the ship's size, the actual volume is.
And in that category it's probably just a bit above the Nova.
The only ship that this defies this logic is of course the T5 B'rel retrofit because it's just a super-buffed version of the T1 model without any physical modifications.
You can point and laugh at me for bringin up an explanation why a larger ship for twice the guns make sense only to have my entire line of reasoning killed off by the retrofit BoP if you wish.
Tell this to the Defiant, which in canon for the time frame it was in was the most powerful ship in Starfleet (and arguably the alpha quadrant). If you watch the DS9 episode where the ship is introduced, you find out how they manage to pack such a powerful punch into a small ship like that.
The analogy of putting a tank turret on a Jeep isn't really fitting, nor much related here considering we're dealing with energy weapons not projectile weapons. Working with that analogy: while you certainly couldn't just drop a tank turret on a Jeep and expect it to work, you could easily take a same caliber gun and build a Jeep sized vehicle around it and have it be very effective.
If you look at modern warfare you'll see that from the time of WWI through till currently, weapons are getting more powerful but they are not getting larger, nor are the platforms that deploy those weapons. Sure, naval carriers have gotten much larger since they first appeared, but the number of aircraft that they support has increased while there has not been a significant change in size of fighter aircraft since the first naval carrier was conceived. If you look at a tank from WWII and a modern day tank, the size is similar but the modern day tank packs way more punch and can withstand way more punishment.
The only thing that really has an impact on the size of a modern day weapons platform is what role that platform is supposed to play and the number of crew it supports.
The reason the B'rel doesn't change in size but becomes more powerful is because the technology has improved while the number of crew the ship supports has not. The ships in the game are getting larger because they are getting more crew. What really needs to be questioned is why they are getting more crew. The role of the original B'rel and the Hegh'ta are the same, but the Hegh'ta has roughly 3x the crew so it's also roughly 3x the size. The reality (if you can really say 'reality' when dealing with an entirely fictional universe) is likely that the ships are getting more crew because they need to be more self sufficient to be able to handle longer periods of time away from a home port.