Tokays can vary a ton in size. Some get that big bulky look. Others are much more petite. Likely due to variations in locale or possible sub species. So you may just have some that are simply not going to get huge. Or, you may be just worrying over nothing. In the wild, there's a ton of variables that affect growth...diet, weather, etc. My advice would be to simply feed them a high quality gut loaded diet, and let them grow to whatever they naturally want to grow to. You say that they're good sized but not "quite there". So it's only natural that they'd start slowing down in growth if they're almost full grown.