i can't draw for shit but i'd say i'm an ok writer all things considered but i currently don't have the time to develop any of these, just putting these ideas somewhere i'll remember them. feel free to use them i guess (also all of these involve muscle/muscle growth but could be adapted)

-magpie grows by collecting various trinkets (the least fleshed out idea but i've had it for ages, this could easily apply to a dragon's hoard (and i believe i've seen a story like that before))

pokemon growing through abilities is an idea i've been exploring, a few are listed here

-dragonite via multiscale (perhaps the mechanics are a rapidly-growing but initially weak set of scales? then a dratini gets separated for being too weak, runt of the litter, etc. or for having multiscale - i.e. its a mutation of some kind? this is personal preference for this idea but the dragonite itself/any others around him are unaware/accepting of its size, they've just always been like that and it never really gets quantified or questioned until he meets another dragonite that's only coming up to his pecs)

-azumarill via huge power (probably not so much growth in this one, they're just Big (thinking like 8-10ft tall) - potential growth triggers: could be them getting caught outside in a rainstorm (i like this one more) or leveling up (and they've still got a ways to go, maybe there's a significant amount of growth and it ends with revealing they're only at lv 70-80? or less growth but they're at lv 30-40? i think incorporating levels makes it start being less of a story and more of a math problem - i'll work on this one more)

-any pokemon via water/volt absorb/sap sipper/flash fire/etc - this is just absorption but still worth listing, specifically note pkmn like goodra, houndoom, ninetails, etc

-mimikyu via disguise - there could be a LOT under that cloth, kinda a free pass for me to write whatever i want - why not let it be able to absorb anything it can fit the cloth under. sure.

-lugia via multiscale - not sure how to do this one. maybe it outgrows those underwater ruins (re: B2W2) or just comes to the surface and starts fucking shit up

-mewtwo, specifically mega mewtwo x: "Psychic power has augmented its muscles. It has a grip strength of one ton and can sprint a hundred meters in two seconds flat!" mewtwo is a relatively easy character to write for since you can write him the way you would probably participate in the story - a self-centered omnipotent god - and it would be very easy to get to the point of this

-any pokemon with pickup - they find a berry (or item - it's just for flavor) that causes growth. they assume it's a one off thing - they grew an unnoticeable amount for 1 and just really liked the flavor, but then they find it again. and again. and again. more and more often until they start finding multiple a day - until the point is reached where crates of it are getting delivered to their house daily

more will come later, these were all off the top of my head and i'll look through the list of all abilities soonish