The 9x21 is a longer case and has more volume, can be loaded to little longer OAL.
(Per SAAMI, 9x19 case is .754" long, volume 13.3 gr H2O; 9x21 is .833", 15.6 gr H2O.)
But, 9mm major is basically a "wildcat" cartridge, so we're not going to see much published load data (if any).
I load 9 maj with 124 gr PD or MG, 8.6 gr 3n38, 1.170-1.175" OAL, SR primers, cases sized with a U die (so they grab the bullet tightly). 171 PF. No problems.