Please allow me to partly disagree. Indeed I find it hard to imagine that in 3301 any amount of recoil a cannon aboard a starship produces cannot be countered by e.g. synchronising the ship's thrusters to accelerate when the cannon is fired. However, in terms of density, ED ships are really light, rather resembling airplanes than tanks or naval vessels. Of course 1300 years of metallurgy/material sciences should have produced amazingly strong alloys and bulkhead arrangements, but still... the ships' structure could be an issue.
It could, and scientifically should. In the end it is about game balance, i guess. Combat in ED is -by design- Battle of Britain in space. So using WWII analogies to guess FD's design decisions could be a reasonable approach.
Just some rough wikipedia numbers for comparison:
16"/50 cal Mk7 naval gun
gun mass: ~121t
105 rounds mass (AP/HE mix): ~105t
battery: 9 (Iowa class BB)
displacement of Iowa class BB: ~52,000t
-> main battery total mass: 2034t (without casings, turret armor, bulkheads etc.) = ~4% of displacement
And from coriolis.io:
class 4 cannon
gun module mass 16t (including 105 rounds)
Fer de Lance mass: ~450t
-> main battery total mass: 16t = ~3.5% of total mass
Anaconda mass: ~1500t
-> main battery total mass: 40t (including 3 large cannons) = ~2.7% of total mass
I did not compare density yet, but I remember there is already a thread on this forum.
tl;dr: I do neither want to criticise nor defend FD's design decisions, my point is just that from the present point of view ED cannons are reasonably heavy for the spaceframes they are built in. Therefore a reduction of muzzle velocity (cp. to high-velocity naval or tank guns) in order to reduce stress on the spaceframe is just as logical as the whole combat model.