How do I increase the storage for my stimpacks and radaways?

In Fallout Shelter, what is the proper way to increase the storage space for stimpacks and radaways?

I have reached a 15 stimpack cap with any further production being discared. The storage room itself doesn't mention stats for stimpack storage cap. The Medbay does have a stat for stimpack storage but it labeled as "10" which is not my observed cap.

So, should I:

1. Upgrade my storage room?
2. Add a second storage room?
3. Upgrade my medbay?
4. Add a second medbay?
5. Something else?

Thanks!