Surely it has to be Big Bottoms by Spinal Tap? Its no more ridiculous than some of the songs above...

The bigger the cushion, the sweeter the pushin
That's what I said.
The looser the waistband, the deeper the quicksand
Or so I have read.
My baby fits me like a flesh tuxedo.
I love to sink her with my pink torpedo.

Big bottom
Big bottom
Talk about bum cakes,
My gal's got 'em.
Big bottom,
Drive me out of my mind.
How can I leave this behind?

I saw her on monday, twas my lucky bun day
You know what I mean.
I love her each weekday, each velvety cheekday
You know what I mean.
My love gun's loaded and she's in my sights
Big game's waiting there inside her tights