The Tamiya Gray, or any other 'lacquer' spot/finishing putty will always shrink as they require the evaporation
of the volatile thinners to harden. Epoxies like Aves, Milliput etc. rarely shrink as their curing process is completely
exothermically chemically reactive, in udder woids the two bits o goo don't do shit unless mixed in the correct quantities.
Too be blunt if Milliput or a similar two-part epoxy compound doesn't harden harden correctly, it's either old (past it's shelf life)
or you've stuffed up the mixing instructions. I can say this absolutely after many years working with epoxy compounds of numerous types in a variety of work situations.
My personal preference is too fill hole/voids with styrene sheets/chunks/bit and then glaze over with an epoxy compound
followed by the various lacquer glazing putty options, all dependent on personal choice, and then a nice high-solids primer
a bunch o' sanding stages.
Don't fear the Stanley knife and the rasp, they are your friends.