I asked my wife and she isn't using any other features besides just plain texting.  She does send a picture once in a while along with an occasional emoji but I don't think that would warrant the large amount of data being used.  What other features are there that use data?

So, the data usage of the app itself I found on the phone by going to settings, apps, message+.  On that screen it shows the mobile data usage.  I text a ton more than my wife and send lots of pictures and my data usage for message+ is only 219 mb.

As far as the timing of the data usage, I found that on My Verizon.  That's where I found that chunks of data are being used around 3:00 am and 9:00 am.  Although, on My Verizon, I can't see the exact app using this data but I'm assuming it's message+ as the phone lists message+ as the only app using large amounts of data.