liability reasons..... what aload of crap (and thats the polit version of saying it!).
The account tranfer only works well if the disk space usage for your whole account is below 200mb. If its more; what you can do is generate a full backup of your old hosting account, make sure you have a copy of it, then delete all the contains inside the public_html folder on the account you want to copy.
WARNING: DO THIS AT YOUR OWN DIGRESSION!
What you can do is then copy back onto the new host afterwards. What this should also do is allow you to copy the account over using the transfer function.
[WHM --> Transfer --> Copy an account from another server with account password]
Please Please Please be careful with this as it is not a 100% working solution, make sure you have a backup of everything before you try.