My head-canon is that the Founders met the Progenitors somewhere along the way and hard-coded a kind of “default” solid look into their collective matrix that resembles the Progenitors. The Founders emulate them in a lot of ways (affecting the evolutionary trajectory of life forms like the Jem’hadar and Vorta) and so “honor” their work by trying to look like them when they take on a humanoid form. Given this, Odo looks the way he does because that’s what a solid form Changeling is intended to look like.
(This is my nerdy way of reconciling that the Progenitor in TNG is played by the same actress as the Female Changeling in DS9 and both have very similar make up effects)
My head-canon is that the Founders met the Progenitors somewhere along the way and hard-coded a kind of “default” solid look into their collective matrix that resembles the Progenitors. The Founders emulate them in a lot of ways (affecting the evolutionary trajectory of life forms like the Jem’hadar and Vorta) and so “honor” their work by trying to look like them when they take on a humanoid form. Given this, Odo looks the way he does because that’s what a solid form Changeling is intended to look like.
(This is my nerdy way of reconciling that the Progenitor in TNG is played by the same actress as the Female Changeling in DS9 and both have very similar make up effects)