Well your assuming that the pouch was made from Hessian boot's that where in good condition and disassembled for the purpose of making a pouch.
When the subject of hessian boots came up here I assumed that it would have been salvaged leather from worn out boots.
That would lend the maker to size the pouch with what ever he had at hand,
The first thing that wears out (normally) on all real leather shoes or boots are the heels. That's part of the reason they were often held on by wood pegs, to make it easier to replace them. After that, the soles are next to wear out. Shoe uppers usually did not wear out unless deliberately or carelessly cut, abraded, heated too close to fire to dry and burnt or were not maintained properly. When not maintained properly, they tend to crack through where the leather is creased by constant bending. So unless the boot/boots suffered battle damage, most of the leather in the boot (outside the soles and heels and perhaps the front ends of the uppers) could have been used to make a pouch even from a "worn out" boot that could no longer be re-heeled or re-soled. The boot uppers would have been in good condition even after wearing out heels and soles, though. So I'm not automatically assuming the boot/boots was/were worn out - other than perhaps the new owner could not get them re-soled or re-healed and/or once those were worn too much to replace - he chose to make a pouch out of one or more of them.
I'm also assuming that IF the boots were in good condition and were taken off an enemy soldier, they would have been used until the heels and/or soles needed replacement and could not have been replaced. Sure, if the boots were too small for one person, they would have been traded or given/issued to someone who could wear them.
Of course a hit from a rifle or musket ball, or cannon ball or shot, or less likely a saber cut damage in battle might well have caused more damage to even an excellent condition boot. Though it is perhaps grisly to think of, the boot from a leg or foot that was amputated could also have been the source of the boot to make the pouch.
I agree the maximum size of the pouch made from a boot that had both the heel and sole worn out was up to the maximum amount of leather that was still "good" to make a pouch. I'm merely suggesting that many if not most of the Hessian boots could have made larger pouches than this one, IF the owner desired to make it to the largest size possible.
Gus