Colony strength, along with colony ship quality is determined by the industries you have. Heavy industries allow for self nade hulls which increases ship quality (less d-mods) and maximum size. You can upgrade your patrol HQs at the cost of an industry slot into military bases, and then into high command, that will make the fleet your colony launches a lot bigger. Another solution is changing your fleet doctrine, which can be done under the command tab on window 4 "Faction Doctrines", you might have way too many skilled officers and barely any ships in your fleets. Tech level on your stations also matters a lot, changes the weapons they have and their overall hp.
If you want to, you can bring a few marines to the hegemony's main industrial world and steal their nanoforge, which will cause their ships to become tinfoil, then install them on your heavy industries to have your ships have 0 d-mods. And an upgrade you can add to your military bases which increases the fleet size are "cryoarymithic engines" or something, they increase fleet size by 25% or 100%, if you have it on a hot or extremely hot planet, respectivey.