Fingers that feel weak are in my experience uncoordinated fingers. So it's not so much strengthening them as in building muscle, as in gaining control over them and moving easily and efficiently. Then you'll feel like you have more power with less effort. Which sounds paradoxical but is true.
I think as bwl_13 says that the technique is developed over years, and that it is not a matter of strength either. I also think it depends on what piano you practice on, electric pianos or keyboards are often very light and you don't have the feeling of weight.For years I have tried those by Hanon, Schmitt, Philipp, Beringer, the Czerny Op. 299 School of Velocity études, and Op. 740. Then the Chopin études. I am 59 years old and I started studying at 5, and I still love practicing the piano.