From other postings, one or two people reported there were problems with colon scarring and the vaginal canal was not made very long.
(One possible reason given was heavy anal sex a while before operation.)
Imo it is possible the tissue in there is stretched to create a cavity.
Maybe this could be talked through before the operation, talking about the risks and maybe rather taking the risks than a shorter canal.
Or maybe a more lenghty operation with an added fee.
There are a few surgeons who offer deepening, for example
marcibowers dot com/mtf/mtf-services/vaginal-deepening/
tmeltzer dot com/mtf-vaginal-deepening.html ( from one posting his price was quoted as lower as a full surgery)
and there should be others... an inquiry might be possible...
Another option would be asking the gyn for estrogen cream to soften the skin, and dilate.
https://www.susans.org/forums/index.php/topic,95839.msg722916.html#msg722916A further option would be to look up suporns manual for dynamic dilation.
all at your own risk... and talk it all through with your doc.
hugs