In order to answer your question, more details would be needed:

1. I assume the two tanks have the same substrate? What is the substrate composed of? Is it a custom mix or a commercial mix?

2. How are they setup? Are the heating elements you use the same? You mention the use of UTH, but is there also a heat lamp to keep it 75-80?

Usually humidity is easier to keep high in a smaller tank than a lower if they were indeed identically setup and the general approach to raising humidity is restricting ventilation, but without more details I cannot offer much insight.