GLA was at gate capacity when it was handling half of this traffic in the 90s, difference then was a higher proportion of small jets and props versus A319/B737 and upwards now. Traffic patters have changed but the number of based aircraft is only getting back to where it was in the heyday of BA. To be fair, outwith the first and lunchtime waves, GLA can be a nice quiet airport.
Two years ago in mid week in mid summer, I was astonished to see only one aeroplane across the whole commercial side, a Loganair SF340!
If Ryanair are serious about GLA, PIK must be at risk, perhaps remaining for maintenance only. Buck for buck, even in a price sensitive market, GLA-xyz is likely to be higher yielding than PIK-xyz.