The smallest the ship, the more time it takes for it to become locked.

Inertia is also lower. (How fast the ship turns and reaches warp speed).

Average sizes are as follows:

Light Drones and Shuttles = 25
Frigates = 40
Medium Drones = 50
Destroyers = 85
Heavy Drones and Fighters = 100
Cruisers = 135
Battlecruisers = 275
Battleships = 425

Capital Ships measure in 1000s, so a lock on them is essentially instant.