What does your vet say? And are they gaining weight overall, week to week, rather than losing it? Some piggies may just be naturally small, especially if they were malnourished as babies- but also there could be health issues going on.
A thorough vet check is the first thing to do, and ask the vet about their weight for their size and heft. With a healthy hay based diet, over time, very thin piggies fatten up, and very fat piggies slim down, to reach the weight that is healthy for them- assuming their are no underlying health issues, like dental or digestive problems.
When we brought home Blodwen from a neglect situation, she weighed 700g at 9 months and couldnt eat hay properly because of serious dental problems- after successful dental surgery she gained 400g in 3 months, just eating hay and a normal diet.
So I would suspect perhaps if you are sure of the piggies ages, there may be a health reason that needs exploring about why they are so small.