How many MB can I send on iMessage? I was wondering if anyone could tell me how many MB I can send over iMessage? And when and why messages get compressed sometimes seemingly by random. I find it very awkward that Apple made this very important information secret and nowhere to be found on the net. 

iPhone 6, iOS 9.x
This is from the iPhone user guide:


  Share photos, videos, your location, and more 
  With iMessage or MMS,
  you can send and receive photos and videos, and send locations,
  contact info, and voice memos. The size limit of attachments is
  determined by your service provider— iPhone may compress photo and
  video attachments when necessary.


You didn't indicate if you needed an answer for wifi or cellular.