Hi,
As others have said they are very small for their age but you are a great and caring owner. Try not to worry, they seem happy and healthy in themselves.
One of my girls was the runt of the litter and was tiny when I adopted her. She was perfectly healthy though.
The rescue were happy for me to take her as her mum had started to get annoyed with her and push her out plus the rescue were confident as she already new me through the forum and didn't live far away.
I fed foods such as carrot (not in large amounts), leafy veg, apple, pepper, cucumber, celery and corn on the cob. Please introduce new veggies slowly. I also sprinkled dry porridge oats over the top of the dry food which all my piggies seemed to love. I *think* you can water the oats down but I didn't as I wasn't sure and they were fine eating them dry.
I feed a combination of nuggets and muesli but as already suggested, excel blackcurrant and oregano and Wagg optimum (which I think is now called Harringtons) are brilliant for small mouths.
Bringing them indoors somehow or at least moving them to the shed will be good. I left mine out but it was a hot summer and she had 3 bigger girls to snuggle up to.
My girl soon put weight on as she grew and is now my second largest piggy