Just make sure you are comparing like to like.
Empty can mean really e-e-m-m-p-p-t-t-y-y or it can mean empty of fluids and ammo but fully equipped other wise. Sometimes called basic weight.
AHT says the Brewster 239 weighed 3744lbs empty. I won't argue over 41lbs weight, some production planes varied more than that from serial number to serial number.
However it added 443lbs of equipment, armament and crew for a two gun fighter and 585lbs of equipment, armament and crew for a four gun fighter (four guns are one .30 cal and three .50 cal guns) Empty equiped or basic weight being 4187lb and 4239lb respectively
By the way the .empty weight of the F2A-3 was 4732lbs so it was only about 1000lbs heavier empty.
And if you compare like to like to like (four guns and 110 US gallons of fuel) the 239 and the F2A-3 were about 1045lbs apart when loaded. Granted you could load the F2A-3 down more.
There are a number of reasons some countries used these "light" fighters. One was that they were already built and paid for which makes them really cheap.
Continued production becomes more dubious however.
We seem to be saying that all fighters are nearly equal in firepower for example.
While a Ki 27 might well be cheap to build based on it's weight (and the power of it's engine) it's effectiveness against certain targets can certainly be called into question.
It used two synchronized Vickers 7.7mm(.303) machine guns not too different from those used on a Sopwith Camel, rate of fire increased about 50%, or about 900rpm not accounting for the synchronization. Compare that to a Hurricane with eight guns. the Hurricane can deliver 5.3 to 6 times more bullets per second. the Ki 27 carries 1000 rounds of ammunition, the Hurricane carries around 2670.
If you are trying to shoot down twin engine bombers with any sort of protection which fighter is actually going to be cheaper? If you need 3 light fighters to get the same amount of firepower into the battle area than the "heavy" fighter then they are not cheaper. You not only have to pay for the planes but you need more pilots and more ground crew.
The F2A1's as received by the Finish had the R-1820-G5 export version engine, hydraulic prop., 3 50 cal. MG and one 38 cal. MG. The 30 cal, was replaced by a 50 cal.MD in Finland. The tail hook, life raft etc, was removed and the tail wheel replaced with a larger (and higher drag) grass field friendly one. The Finnish also added piolet armor (do not know weight) and a German made reflector gun sight. In another thread on this forum is a detailed weight break down of weights for for almost all the Brewster's except for the 239. A significant part of the weight increase for the British planes was increased ammo capacity.