It also implies that there's not enough power on larger ships.That implies that agility works by moving the targeted hull (or specific parts of it) out of the weapon's area of effect.
Simply, the reason smaller craft are "more agile" is that the power/weight ratio is higher than a larger ship.
But consider, something like a Saturn V, which by all accounts is pretty large, accelerates faster than most ground cars. What it lacks in nimbleness is more a factor of that it doesn't have any wings to make it turn (in atmosphere) like, say, a missile does.
But give it suitable vanes (and the structural integrity, I'm going to bet as is, the V doesn't really turn well), the V will turn quite quickly.
Put a 6G 500K ton dreadnought in a race with a 6G 50 ton cutter, and guess who wins the race?
The singular problem with larger ships is that they have more surface area they need to "get out of the way" compared to a small fighter or a missile. (Which is where size modifiers come in to play.)
The other issue is simply that we don't have much information on how ships use M-Drives to turn, rather than just accelerate. They don't have rudders, they need more like bow thrusters to make turns, or maybe REALLY BIG gyroscopes. And the power of THOSE will actually have more influence to the "agility" than the M-drive alone.
A 6G ship with paltry .1G thrusters aren't doing any turns very quickly. Like the Suzuki Samurai, "it really goes straight!".