Well, sort of. It is not so much about reducing weight or size; the rover weighs 900kg (basically, one US ton). It is about energy budget; with the radioisotope thermoelectric generator, "nuclear battery," there is ample power not only to move such a behemoth around, but also to power some really interesting scientific instruments. For example, the rover has a laser which can vapourize a small sample which can then be analyzed spectrally to get chemical composition. Imagine trying to power a laser like that on solar panels.

In space, in the inner solar system, solar panels are more power dense than RTGs.

The situation might change enough being on planet where the sun is slightly filtered by the thin atmosphere, and the sun doesn't shine ~12 hours a day.

RTGs have many other advantages: they provide heat as well as power (which is really important on Mars) and they don't wear out when covered with dust. If Curiosity stops running within a decade, it won't be because of power.