I'm with Ric.. we bought some weighing scales a while ago and never looked back! We thought some of our animals weren't growing too quickly, until we starting taking measurements of length and weight and you would be amazed how much they grow without you noticing! It's often the same as anything.. if you own something and see it every day you don't notice the changes, but if you haven't seen something or someone in a while the difference in appearance can be amazing!
I also wouldn't worry too much - as long as you are feeding him the correct diet and he is eating and defacating normally, I'm sure he is fine. Remember, as with all living things, reptiles grow at different rates; one gecko may shoot up in size over a couple of months while the other doesn't seem to grow very much. Everyone and everything grow at different rates and not all grow to be the same size (i should know!!

) When you read, "x lizard grows to an average of 12inches", it is an average,. that doesn't mean that some won't only grow to 8" while others might double that.