Some are just smaller than others - One of my boars is just under 1kg at 3 years old whereas I had another who was 1550g!
Weight is not the gauge as to whether he is a good size - all weight does is tell you whether he is eating enough hay (hay being the most important part of the diet, not veggies or nuggets) to maintain himself each week.
The way you tell whether a piggy is a good size for themselves is to check heft. You put your hands around his ribs and if you can feel the ribs but with a thin layer of fat over them then they are a good size. Can’t feel the ribs at all means overweight; can feel all ribs and spaces between with no fat means underweight
I’ve added a guide below
Weight and Weight Loss Explained: BMI, Weighing, Poos and Feeding Support