The first thing to remember is that these are the lyrics to a rock/pop song. Lyrics are a form of poetry, and pop songs are often considered to be written by poets who have less skill than Blake, Wordsworth, Dylan Thomas, etc. With the poet possessing less skill, some song lyrics can be awkward, especially when the need is strong to choose a word that rhymes.
The second thing to remember is that this is a film by comedians who are writing the songs to parody many aspects of rock bands, including the crude poetry in their song lyrics. This makes the Spinal Tap song lyrics more likely to have odd imagery or phrases - exaggerating the bad poetry to heighten the humor.
The waistband of trousers/pants would be tight over someone with a large derriere, but only if the person had outgrown the pants. I.e., if the trousers were the wrong size. However, if they are the right size the waistband won't be tight. With women's clothing, the expected body shape has hips and bottom wider around than the waist (the traditional "hourglass" shape), so trousers that are big enough for the hips could be loose around the waist.
Is the singer of the song accurately describing the fit of his lover's clothing? Not really, but remember my first and second points above.
Regarding your question about the singer not understanding skinny girls, this isn't the case. The singer has a preference for women with large bottoms and has written the song extolling his desire in detail. "The deeper the quicksand" is expressing the strength of his attraction to this part of the female anatomy. Many songs express similar helplessly strong attractions using the metaphor of how a magnet attracts steel. This parody song uses the metaphor of quicksand taking hold of the singer and pulling him deeper.
