Hi and welcome
Please weigh your guinea pigs once weekly and combine this with a body check as their life long health monitoring routine. Once monthly is too little. You cannot control the hay intake with your eyes - and it makes around 80% of the daily food intake. Your piggies can lose a lot of weight rather quickly while still begging for their veg; it doesn't happen all that rarely!
Your girls may be fine or they could be too heavy - I have had both at this weight. It is the 'heft' or BMI size/weight ratio that helps you work out whether your piggies are overweight or not. The weekly weigh-in monitors any chances once you know that they are in the right ball park for their individual size. How big they are doesn't influence the life span as long as they are fine in themselves. Guinea pigs have the same taller and smaller people like we humans.
Please take the time to read our weight and diet guides so you can learn how to feel around the ribcage whether your piggies are actually a good weight for their individual size and sort the problem with a good general hay based diet with a small healthy selection of veg that relegates calorie bombs like carrots and fruit to the category of special treats and reduces fattening pellets to 1 tablespoon per piggy per day. Also remove any bowls between meals so your girls are encouraged to eat as much hay as they can and don't stuff themselves with lots of sugary and fatty calories by hogging the majority of the pellets.
The correct diet can make a real difference in terms of long term health and whether your piggies live to the lower or the upper end of the average life span for healthy piggies.
Keep in mind that guinea pigs have evolved on a diet of dry and fresh grass with some herbal forage added in to supplement trace elements and minerals. The closer you can keep your diet to that, the better. Instead of lots of treats, turn exercise and feeding time into an enriched event that your girls have to work for and that is also fun for you to watch!
It is really worth reading these guides here. They contain all the practical details and ideas you will need and that I can pack into one post:
Weight - Monitoring and Management
Guinea pig body quirks - What is normal and what not?
Long Term Balanced General And Special Needs Guinea Pig Diets
Enrichment Ideas for Guinea Pigs