If you are concerned about their weight then that is something to raise with their owners when they're back from holiday. Don't put them on a diet or try and start exposing them to things they're not used to while they're with you. That's definitely something for the piggies mum and dad to take care of
It's lovely that you're concerned for them and want to do your best for them but it's not really your place to change the care routine for someone elses animals.