I think I've figured out what 'pear-shaped tones' are all about.

If we sing with the backs of our mouths open, the tone is fuller. However, we can't just open the backs of our mouths just by trying, we need a metaphor to help us out. If I pretend that I have put a pear (a small one) in my mouth, big end first, then the back of my mouth has to open up to accommodate it, while the front closes down to help with enunciation. Trouble is, I soon begin to feel like I can't breathe - even though the pear is imaginary.

I prefer the metaphor that I have a forkful of hot food on my tongue and I'm opening my mouth inside to cool it off.