I think the quote from Ronald Moore explained it well.
Odo was copying Dr. Mora as best he could. Remember, Odo is practically a infant or toddler compared to most other changlings.
When Odo first returns to the changling home world they probably took a form similar to him (with genders to match him and Kiera) in order to make him feel at home with his own kind. It's always easier to relate to people who look like yourself. They probably respond that way to any changling that returns home.
After that, it makes sense for them to keep that form especially when dealing with Odo because they want him to stay with them and they want him to feel guilty for betraying his own people. and aside from infiltration, it makes sense to keep a consistent form when dealing with other solids. Otherwise people would get confused, not least of which would be the audience. It doesn't make much sense to talk to Odo looking like him and then change into a perfect looking solid to talk to anyone else.
On top of that, the changlings look down on solids as inferior species. So they wouldn't be inclined to mimic them consistently (except of course when it benefits them in some way to do so)
Now the question is why it's so hard for Odo to mimic a face, but he can mimic any other solid object perfectly, like when he becomes a bird. If he didn't do it perfectly he wouldn't be able to fly.
And speaking of flying, it makes relative (in sci-fi land) sense that he can change shape (he's technically a liquid), and size (by altering his density) but how can he change his mass? He's not gaining and losing atoms every time he shape shifts. So if he shape shifted into something really big, he would become less dense than air and would float away, and if he became really really tiny, he would become a black hole.
They could put one sentence in 7 or 8 seasons giving some sci-fi reasoning (like they do with all the other impossible stuff in Star Trek) and I'd go along with it. But every show and movie that has someone changing shape/size seems to skip that part.
When we don't understand how something works it becomes basically magic rather than science (or science fiction) and that's not what we want out of a trek show.