Traditionally, the Shapeshift power let you take points out of Strength, Dexterity, etc, to transfer to powers so that, e.g. your Housecat form wouldn't be as strong as a human. It's up to the GM to decide exactly which traits to allow and how far to reduce the stats.
You can find the Shapeshift power description in all of the core rulebooks as a Sample Power built from the base power of Variable. The description is as follows (emphasis mine):
You can transform into different forms, gaining the physical traits (abilities, skills, advantages, and powers) of the assumed form. You gain (Shapeshift rank x 5) character points worth of traits. You can also redistribute points spent on your own physical traits (lowering your Strength to apply those points elsewhere, for example). You are limited to the inherent traits of the forms you assume and do not gain new mental traits, even if that form possesses them.
As a side note, Shapeshift does not, in and of itself, include any sort of Morph effect so that you change appearance. That's considered entirely up to GM purview as to whether you make your players spend points to not be recognizable when they Shapeshift. Personally, I think that it falls under the "no one recognizes Superman as Clark Kent" situation and they just don't get the ability to look like a particular cat, pangolin, or xenomorph when they change shape.