Oh, Caro, I apologize for intruding on what is really YOUR thread.
But I cannot resist, Jim and Bob:
Although I know nothing about Christian gospel, I think I can get to the bottom of this:
There is no diminutive form for either father or mother. Sometimes people add on, like : Abbale or Immale or such, but it isn't very common.
The gospel sermon I think has to do with the basic Jewish concept that G-d is both King (and Judge) and Father, and we pray to him as both servants and children. These are two different types of relationships, one with emphasis on fear, the other on love. (This concept is stressed in the prayers of the High Holidays - Rosh Hashanna and Yom Kippur.) Probably the gospel wanted to stress the Abba part, as a more familiar relationshp with G-d, and that's why it may have seemed like a diminutive form.
BTW - Abba does have aramaic origins - the ending of the word. It's THE father. (Av is father.) Just like Imma is THE mother.
Hope this helps,
Penny