You might confuse the concept of "solid drawing" with "detailed drawing" or realistic drawing".
Solid just means that the form can be moved well in 3 dimensional space. All the details fit snugly onto the form.
The eyes are in the right place on the head, the face is in perspective....the features don't float around as they do in Terrytoons, Shamus Culhane cartoons and modern Disney fully animated features.
FORMLESS CHARACTERS FROM THE 1940S
In the 40s, everyone copied the Disney/Warner's style - a style made up of simple yet solid constructions. But not everyone really understood it, so there were a lot of sloppy mushy cartoons made by the lesser studios.
No form in any of these drawings.
Some of this sloppiness could be attributed to bad inbetweeners and clean up artists; it's hard to know without seeing the actual animators' drawings.THE MORE DETAILS YOU HAVE THE HARDER IT IS TO ANIMATE FORM
The more details you have, and the more realistic your proportions (tall characters with small heads) the harder it is to control the solidity of them. 30s and 40s cartoons evolved a drawing approach that made it easier to control your details and forms in 3 dimensions - but you still have to understand form and hierarchy or your characters will melt, like in late 30s early 40s Walter Lantz cartoons.
Here's the gay lead in Disney's "Pleasure Package" who can't make up his mind whether to marry the big burly brute
who beats him - or his Mom. (who's the same age as him)
(I won't give away the ending)
The animators had a hell of a time turning the character's head in space because he didn't have a solid construction. The angular planes at the top of his head didn't match the perspective of his head positions. His facial features aren't anchored solidly on his face. His nose is too low and too vague. 
His teeth aren't set into his jaw, so his mouth just floated like a magic hole on a sea of flesh color. - like Anime characters, only not so stylized.
Watch a clip of the character moving if you have this classic in your library. You can see his features floating all over the place. His face is constantly trying to find where his head is. I find this extremely distracting and I can't follow the importance of the storyline of whether he will stay with the butch or leave to marry his Mom. (I won't give away the ending)
On another note, would you hang out with this guy?
Bob McKimson was the king of solid drawing
McKimson was so good at drawing anything from any angle, that he didn't have to rely on animation tricks to move from pose to pose. He could move slowly, directly and naturally - moving his heads and bodies into very difficult to draw angles. His characters move less like typical squashy stretchy cartoon characters and more like people. Clampett was able to act out human scenes to McKimson, who would memorize the acting and just draw the scene straight ahead, with no melting and no reliance on animation tricks.
Milt Kahl is also a very solid animator, but it's a bit harder to see in motion because he does use a lot of the other Disney animation tricks.
There is a lot more to say about solid drawing. To me, it's the most important principle of drawing there is. It seems to be completely out of fashion today.
ONE LAST THING
