isn't funny how 2 children 4 years old can be different sizes too, and 2 adults 40 years old can be different sizes too.
A full grown corn snake can be 3 feet long and 300 grams, or 6 feet long and 1000 grams. Just because 1 snake is bigger than the other does not mean 1 is underweight.
If you're going to be a serious snake owner and you want to do things right, get a scale and weigh the snake(s) regularly. That's the only way you can tell if they are gaining weight or losing weight and by how much.
Most of us all weigh our snakes regularly and we weigh the food items too, so we can be sure we are not over feeding or under feeding.
None of your pictures look like an underweight snake to me, but your pictures are not good quality, so I'll just stop there.
As far as proper feeding, even if a snake is underweight, you don't necessarily double its food. Snakes can get obese too just like people. The best thing to do is to feed on an established schedule and stick to it. Do everything repetitiously, like a habit. If a snake is underweight, it can and will catch up over a period of time with proper feeding.