You can get an idea by looking at a hatchling that an animal will develop a large head as an adult but it's not a hard and fast rule.
Sometimes though it's not till right around the time that animal hits maturity that the head structure sort of "explodes". From my observations giving the animal plenty of time to mature will result in better overall structure and just a more “mature” look. Just because an animal physically CAN produce offspring doesn't mean it's the optimal time for it to produce offspring. :) Another thing to take into consideration is the parents structure. Structure is something that can be improved on fairly quickly by incubating the eggs cool so that they incubate longer. This has been observed to result in larger, stronger hatchlings with better/larger head structure. LONGER is BETTER with incubation. I aim for 90 days at least which is achieved here by incubating at room temps of about 70-72. Starting with a well structured adult is going to get you there faster still and should ALWAYS be the goal.
This is a clutch of babies I produced years ago out of the above pair. As babies you see that their crests were kinda...weird. It's like they had “bed head”. Most, myself included, would not have looked at these two animals and expected them to be large adults with large heads...
This is the same two animals right as they were first paired. Ginger was 45g and Bentley was 40g. I also personally believe that giving them a chance to be FULL grown (as in not actively growing anymore for several months in a row) before pairing makes a big difference on reaching their full potential. They had reached that goal. Their structure was good. They looked like mature animals and were over 2.5 years old and ready to be paired.
This is them (two years after being paired) as fully matured adults. You can see that they filled out even more after being paired and my theory is that the hormones of breeding may give them a last little boost of maturity.
Sometimes people who are eager to pair an animal will pair before they've had a chance to FULLY mature only based on them reaching a “minimum” age and weight. I have long wondered if that at least somewhat stops them from hitting their full potential. Ginger and Bentley shown above were given plenty of time to grow before being paired and it still wasn't until into their first breeding season, that their structure FULLY matured. Moral of the story? Don't rush it. Waiting a bit longer won't hurt one bit and might just get you the larger, well structured adults you're wanting. :)