Ever wonder how big a Wailord is compare to something like a Palkia? Ever stop to think about how small a like Castform is? Then we suggest you take a look at a piece done by DOTBstudios.
The image above contains all 718 known Pokemon as of right now including their Mega Evolutions. Some Pokemon like Venusaur actually do get bigger upon pressing that Venusaurite on your Mega Ring.
Click on the image above to enlarge it.
Source: Anime News Network