while a botanist may be of some assistance, a dendrologist is much more helpful.
first, let me start by saying that I am not a fan of P. calleryana (Bradford Pear). As it ages, it increases its chance of splitting. I have witnessed many larger specimens almost splitting in two during a strong storm.
Regardless, almost any tree will "widen" when the apical meristem is cut. This is the terminal bud of the highest reaching branch. If one were to cut the "top bud", the other branches will "race" to become the top bud, while other branches will form (usually giving the tree a wider appearance) to join the race. This process can be repeated over and over again until the desired appearance is reached.
I don't think this is a good idea for P. calleryana. As stated above, the tree is already susceptible to splitting with age. Adding more growth/mass (thus weight) to be supported by the main trunk is not a good idea. But, if you have to do it....give it a try. Let me know the results!
my favorite way to trim a bradford pear is with a chain saw, trimming 2 inches above the ground. I have had every bradford pear on my property split within the last 3 years and decided to get rid of all 6 of them.