Most of the Megachurches are preaching sound doctrine. That is why they have become megachurches.
Size is no measure of sound teaching--if so Full Gospel Central Church in Seoul Korea(hundreds of thousands of members) would be the standard of the world--it has many doctrinal problems.
Sorry, but the reason some have become megachurches is that they have followed certain principles -- principles straight out of Harvard business school -- on how to "succeed" as an organization. The scandal of the megachurch movement is that it is possible to become big, without the necessity of following the Holy Spirit.