Problems downloading from Android Market - related to package size?

I regularly get support emails from users who can't download my app from the Android Market. They say that after purchasing the app, it fails to download.

The only thing I can thin开发者_JS百科k of is apk size: mine is 30MB. My standard reply is to try again on Wi-Fi, but I don't think this has fixed the problem for a significant number of my users.

Specifically:

• Is there anything other than apk size that could cause this?
• Other than slimming down, which I'm planning to do eventually, what might help?

I've seen this issue before and usually if I select Market in my Applications list and clear the cache it will fix the issue.

30 MB is a huge download (i think that's the apk limit) so i would definitely implement some type of in app download to sd card like most games do.