It's somewhat strange to see Ryanair expanding during the winter but somewhat stagnant during the summer when demand should be highest. I always feel that Ryanair has never reached their full potential from Bristol and I'm not sure why.
Ryanair's small increases in Summer 2018 is not only at BRS but across the UK with a few exceptions, so I wouldn't worry!
In Jan-Mar 2018, out of all RYR UK bases (not including STN), BRS saw the biggest increase in number of departures in comparison the same period last year. Quite impressive really! Only EDI and LBA saw increases during this period too whereas MAN, EMA, BHX, LPL, LTN, GLA, BFS, BOH all saw decreases and PIK no change. Out of these decreases, some were big (LTN, BFS) and some small (LPL, GLA).
From Apr-Jul 2018, out of all RYR UK bases (not including STN), only EMA, PIK and BOH have seen decreases in the number of departures in comparison to the same period last year. The rest of the UK bases have all seen increases, the biggest of which have been MAN (+659 more flights during this three month period) and EDI (+531).
GLA has seen big increases too (+186) but not to the same scale as MAN and EDI.
After that, BRS sees the biggest increase (+63) followed by LTN (+57), LPL (+50), LBA (+31) and BHX only (+11).
Hope that gives you some insight as to Ryanair's current operations. It does seem that they have not really been increasing any of the UK bases apart from MAN and EDI during Summer 18 (which have both seen an increase in the number of based aircraft) with all other bases having only small increases if any. Winters seem to be where Ryanair are making the largest increases and BRS is no exception this coming Winter!