We recently had a customer who had a 55 MB mht file being created for his Active Report. The report had two level of drill downs with charts and crosstabs on the deck. This kind of size is generally not recommended for Active Reports, especially if the Active Report is to be used on an iPad. It is important to note that an iPad is a relatively small form device.
On further analysis it was found that there were many charts in the .mht with no data and many of them were inaccessible by the end user. This was pointed out to the team on the ground. They then tweaked the queries driving the card creation and the file size was reduced to as little as 8.5 MB.
These guidelines were used by the team on the ground.