You have two choices, you either embed the resources in the application and it grows large, or you don't embed them and have to serve them at runtime.
Serving them at runtime means your user will have to sit and wait a second time (after installation) to use the app, you'll need to deal with partial downloads as they inevitably close your app since it doesn't work right away, and you'll need a place to host the data.
Embedding them means it can't be downloaded over 3G, so users will need to be near Wi-Fi to download it.
Embedding them is the way to go. 20MB is nothing over Wi-Fi, and if someone can't be bothered to wait and hop on Wi-Fi to download your app, they probably weren't too enthused about it in the first place.
This isn't even considering the fact that you say it's an iPad app, the majority of which do not have 3G connections anyway.