I don't fully understand your question. The word is mfereji, not mferegi, right? And I don't think there's much of a difference in how the letter J is pronounced between the words in this course, at least the ones I have seen so far. I think it's always the same pronunciation, like the first sound in English jig.
EDIT: I actually got an audio exercise for this, now, and the recording does make it sound more like mferegi, this time. Perhaps just the recording being slightly off? I don't think it's meant to sound that way, anyway.
In English you have a lot of words for "big" things. Tall is used for height only. You would say a tall man, not a long man. Roads, canals, railways are long not tall. With words like big, large and great you have similar differences. Large is used for surfaces, big for three dimensional size, great is used for for non physical size Bach was a great composer