It must be understood here that simply dropping a smaller wad into the larger shell is not the same as firing a smaller shell in an adapter. The wad "Fits" the smaller shell & has its pressure up before exiting the end of the case so it immediately expands. Just drop that smaller wad into the larger hull & seat it & you will have powder outside the wad to begin with. Touch her off & you are obviously going to get immediate blow-by & the wad will most likely never obturate efficently at all.
"Totally" different circumstances.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra