You can increase the size of your weapon safe by talking to a woman next to the blacksmith at night time. She will increase the size of your safe for the following prices.

Increase from 20 to 30 slots: 500 Mon
Increase from 30 to 40 slots: 1000 Mon
Increase from 40 to 50 slots: 1600 Mon
Increase from 50 to 60 slots: 2400 Mon
Increase from 60 to 70 slots: 3200 Mon