There's a very simple, and time-saving way to do this achievement ... but you need to knock out some whims first (the thought bubbles above your Sim's head in the bottom right).
Once you clear a few and have 1500 Aspiration Points, you're set.
With your points in hand, go to the Aspiration Tag at the bottom right (the hexagonal box with a gold icon inside), click it and locate the Present icon in the box that appears. This is the Rewards Store: Click this, and you'll find a list of potions and extra traits.
In these potions, you can find Insta-Large and Insta-Lean. They cost 750 points each. Buy both, pop into your inventory (the box icon also located in the bottom right corner) and drink both potions. Once your Sim downs the drinks, the achievement will pop.