Is it possible to increase the capacity of Stimulants and Antiradins. (Fallout Shelter)

Yes, improve the rooms where they are made, thereby increasing the storage.

SEXXY
You can also build new ones - it is cheaper but uses energy

The upgrade affects only the amount of antinadin and stimpacks produced at one time, the capacity is affected by the combination of these rooms. I built them, combined them, did not upgrade and left them empty except for 2 in which resources were produced.