The more wattage a heat has the bigger it is?

Currently i have a RUB set up with a i think a heat mat of 7w and a distance of more or less 3 cm between the mat and the bottom of the RUB.

The mat is not giving off much heat, not enought to pass trough the bedding (2cm deep aspen).

Ideas:

Thin the bedding depth in the hot spot?
Get a bigger (more wattage) mat?
...

Thanks