I did a little bit of searching and I couldn’t find anything and I don’t have an intuitive answer either. :P The baseball effect is the main way I’ve heard of the shape of your penis changing. Perhaps clamping? Some people have gotten girth gains at the base larger than girth gains in the middle from clamping at the base, but maybe just straight up jelqing would do it too. Or maybe your large head to shaft ratio will stay the same no matter what. I would just do length and girth exercises and see what happens (even if you don’t want length). If you want to get gains and have yet to get any, I don’t think you can get too picky. I also think a larger head is awesome for sex and can be used to great effect to make a girl go crazy in bed, so I’d be happy with an extra large head if I was you. Make sure you use plenty of heat too if you want gains.
If you’re really curious, perhaps this question would be better addressed in another thread and some of the more knowledgeable members would have a solution.
Starting: 5.9" BPEL, 5.5" EG
Goal: 7.0" BPEL, 5.5" EG
"Being natural is simply a pose, and the most irritating pose I know."--Oscar Wilde, The Picture of Dorian Gray