How DO they stay so fat?

I've been evicting some piles of old "Bubble-wrap": greenhouse insulation for the use of. In amongst it I found at least 3 of these. One "took off"/escaped by accident the previous day, leaving a known 2. "Today" I removed the last batches pending a run to the tip to recycle, still at least three HUGE, FAT, residents.
This bloke/blokess dropped out of the last bundle and played Possum for a couple of minutes prior to legging it into the undergrowth.
I wonder if it'll get back in, I think I'll leave the door open when warm.