Voracious Wurm

Cast it cold on turn two with an empty life-gain ledger and you get a bare 2/2; cast it after a painless gain spell or an incidental drain trigger has nudged your total up, and the replacement effect finds a number to stamp on. That "enters with X counters" text is the whole design axis: it is a replacement effect, not a triggered ability, so it never touches the stack. There is no window to respond to the sizing and no way to interact with the growth once the creature is already resolving. The game checks the running total of life gained this turn and creates the object with that many +1/+1 counters already on it. The distinction matters afterward, too: because the bulk lives in physical counters rather than a static "gets +X/+X" bonus, it survives effects that would peel a printed buff back to baseline. This belongs to a line of green cards that asked life-gain to buy board presence rather than a defensive cushion, treating gained life as a resource to convert into stats. The honest constraint is that it generates none of that life itself, and it counts only gains, not any losses your manabase or your own spells might incur along the way: a payoff, not an engine, only as large as the turn that feeds it.