I'll admit I do weigh when they're not empty too! I only take their empty weight into consideration for weight monitoring though

Age is a bigger factor, weigh after a bowel movement and jot it down and then weigh after the next etc and you'll see if they are gaining weight health isn't all about weight as long as they look healthy and they're eating

Don't go solely on weight, if they look healthy I'm sure they are. If you are concerned post photo's and you'll get the advice you need.