1. You reach further down the intent curve
The cheapest, most qualified demand gets harvested first. The next euro is always slightly worse than the last, which is why efficiency softens as you scale. That is normal, not failure.
2. The bidding re-learns
A large jump moves the campaign’s data enough that performance is unreliable for a period. Raise in steps.
3. Weak structure becomes expensive
Problems that were tolerable at €50 a day are costly at €500. Fix them before, not during.
4. Operations get tested
Stock, fulfilment and support. Doubling orders you cannot ship is not growth.