I think I have a lead to guess it. Disc loading.
An 188 cm diametre fan goes into a square 3,5 sq m area. The circle is of course smaller area (pi/4 times factor) but comparing apples with apples, we should be able to eliminate this.
So, we get 3 tons per square m for C130, and less than this (2,7 tons) for C110. MRJ-s 142 cm diametre fan goes into a 2 square m square, so this is from 3,3 to 3,8 tons per square m. Hm, note that there is a pretty wide range, and that Cseries has clearly lower disc loading than MRJ.
For comparison, the 174 cm disc of CFM56-5B3 goes into 3 square m square, and has 15 t thrust. So, about 5 tons per square m.
Going by 3 t per square m (the higher thrust of Cseries) would give 6 square m for an 18 t GTF. Which means about 245 cm fan diametre.
Far bigger than A321 engines, or B757, or the CFM-s of 340-300. But pretty nicely comparable to the Trent 500 of A340-600. And I understand that the underwing space for roughly 248 cm fan diametre of Trent 500 of A340-600 or Trent 700 of A330 is also present on A340-300.
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