(Self-answered question, as it may help other users)
Edit AndroidBootstrap project > src > com.beardedhen.androidbootstrap > BootstrapButton.java:
Replace
layout.setPadding(0, paddingB, 0, paddingB);
By
layout.setPadding(0, (int)(paddingB*0.7), 0, (int)(paddingB*0.7));