Welcome to the forum
Guinea pigs will try their luck - if they learn to manipulate you for more than they need, then they will!
The only food they need constantly is hay. If they always have it then they aren’t hungry!
Hay is the main food source. It needs to be constantly available and never run out as it makes three quarters of their daily food intake. It is that which they must fill up on.
Fresh grass can also be given (but introduced slowly if their gut isn’t used to it).
Veg is one cup per pig per day.
The four safe daily veg are lettuce, cucumber, pepper and coriander.
Fruits and carrot are not needed in the diet but can be given in very small amounts but only very occasionally
High calcium items (such as parsley, kale or spinach) need to be kept limited to one small amount of one type and only once a week
Pellets should be kept very strictly limited to no more than one tablespoon per pig per day. Make sure to use a tablespoon measure so that you don’t inadvertently give too much each day.
Pellets are the one part of the diet which is not actually needed (I personally don’t give any pellets to mine).
Overfeeding pellets can contribute to a variety of health issues: if they overeat pellets and under eat hay then they can struggle with poor dental health, poor gut health (due to not enough correct fibre levels) and due to the high calcium levels in pellets (pellets contain more calcium than the highest calcium veg) they can contribute to the formation of sludge and bladder stones.
You will only know they are eating enough hay via the routine weekly weight checks. Hay intake cannot be gauged by eye but because it has to make the bulk of their diet, if veg and pellets are kept at the strictly controlled levels, then hay is the only thing keeping their weight up (which is what you need)
I’ve added a guide below which will help further. It shows a sample plate of veg amounts
Long Term Balanced General And Special Needs Guinea Pig Diets
I’ve also added our weight management guide
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support Levels
. Any advice welcome
Guinea pigs are greedy little animals who like to try their luck 
don’t let those little faces fool you x