Disagree with Symphony, in that drednoughts are the end game ship.
End game. Yes, that's why they showed up in barely over a dozen turns. You could see them in 10 or less if you honestly put your mind to it with NiNES technology. It could be more here depending on what BananaLee has done.
And like TerrisH says, the idea was never to have Dreadnoughts be the solution to all problems. I do consider ships to have strengths based on their (base) PP cost, but there is also the cyclic element in that 2 cruisers beat 3 destroyers (both forces costing 48 PP), but where fighters have unproportionally large strength compared to their cost, unless faced with corvettes (which will be taken by frigates, etc).
Let us assume all ships are spheres, for easy calculations. Let us say a Battleship has a radius of .5km (500m) and a Dreadnought has one of 1km (1000m), arbitrarily. The Battleship will have an internal volume of 523,598,775.6 cubic meters [(4/3) x Pi x r^3) and 3,141,592.654 square meters of surface area (4 x Pi x r^2), while the Dreadnought will have 4,188,790,205 and 12,566,370.61 respectively. In other words, exactly 8 times as much volume and 4 times as much surface area for doubling its radius. It will also weigh 8 times as much, assuming it's twice as big.
I wonder what most of that additional surface will be covered with? Point-defense guns and anti-capital ship weapons perhaps? It's so big you can have armored sublayers. It's also so big it can afford a large number of additional powerplants to power all of that. So, in effect, it is harder to kill, being exponentially bigger, and simply
thicker--it takes much more ordinance to get at anything critical--and it can mount exponentially more weapons to keep you from getting to do that
Now ships are not going to be spheres, contrary to what the Borg may profess, for various reasons. But bigger ones will be vastly better armed, armored, and larger. Fighters and other ships, being smaller, do not have the kind of energy reserves necessary to pierce that sheer additional bulk. Fighters in particular don't as they can't even go FTL.
If you assume they somehow do have a weapon that can pierce kilometers of armored bulkheads, then they instead are the ultimate ship in the game, because then they can just operate in wolf-packs and saturate areas of space holding corvettes with these magical capital-ship killer weapons, unless you want to magically justify that they somehow don't work against corvettes.
This isn't
Star Wars. There isn't some obvious 1-meter vent port that will blow up the giant ship. Big ships have big armor, big guns, lots of guns, and big power. Small ships have small guns, small armor, and small power. Big ships swat down small ships and small ships can't do much more than annoy big ships, or if you have the Dr. Device or something, it's the other way around.
The only way Fighters can fell Capital Ships with some stunning awe is in one of three instances: 1) you have extraordinarily incompetent ship designers, 2) fighters have superweapons that make them the best ship, or 3) you're willingly fooling yourself into believing they can because that's why sci-fi at large tells you.
Jankenpon doesn't exist in space unless you make it do so.
Now, if you want to do that, great. But don't go pretending it's justified by anything other than being the way the game is built to run, and that it's a completely arbitrary and imagined way of balancing it to boot.
Oh, and as an afterthought: By the logic of Fighters being disproportionately strong against capital ships by all this magical power, all you need then is Fighters and Corvettes to mop the floor with Capital Ships. The system doesn't work when you only need 2 out of 3 options to shaft the 3rd.