Mardle 
Did you used to live in Liverpool?
'I once had an aerobics instructor who was really quite chubby, compared to most fitness instructors not huge or anything, but I think you'd describe her as plump or a bit podgy. She did class after class of high impact aerobics, day in, day out.. But she had the figure of someone you might assume was a bit lazy and ate too many biscuits while moaning about being a bit too fat. I used to look at her and think:
a) how in a fair world, does this work? Why is she not a beanpole? I am exhausted after one class, she'll do three or four or maybe even more in a day.
b) if this is her size/shape given the job she does then what's the point of me flogging my guts out three times a week? '
I swear I had an aerobics instructor exactly the same!!! 
I attended 3 classes per week. This woman was always the instructor. She was leaping around for England, yelling into the microphone too. She was extremely energetic, never stopped.
If you would have met her and she would have said what she did for a living, you simply wouldn't have believed her.
I remember thinking that she couldn't possibly eat enough to match her calories burnt.