If the correct dose for retromutagen is one drop, why do they keep using more?

[neatoman]

I know some people try to claim that Donatello didn't know what the appropriate dosage for retromutagen was when he made those two first canisters and acted as if it represented two doses, which I don't really accept that but you could at least make that argument for that particular episode... But why do they continue to act as if the correct dose is more than a drop? There are moments like Bebop and Rocksteady bickering over which one of them should get demutated with one single spray bottle and Donatello making rather large retromutagen grenades in the season 4 finale. If we know the dosage is just a drop, why did they continue to act as if more was needed?

[IMJ]

Maybe dramatic effect mostly? The sort of moment where it's established that a "Drop" does the trick and so in a scene where someone "chugs down a tankard" of it puts shock upon the viewer. That sort of "he's over the edge, what is going to happen?" moment.

Maybe character effect - Bebop and Rocksteady aren't as calculating as Krang or Shredder so they get a hammer and hit everything in sight whereas Krang hits the head of the nail that he needs to hit.

It's been so long since I've seen the show I'd need to revisit the episodes you have cited to decide whether or not there was any additional story context to the "volume of mutagen used".