Both carriers have been incrementally increasing capacity/frequency for a while now.
KLM have been toying with 6x lately with an additional late afternoon service on a Sunday only. They tried this a couple of years ago but pulled it back shortly after but what with the stronger economy now it will work better I suspect so could be on the cards for S15.
Frequency is more important than capacity really for a hub carrier which is why they don't just run a couple of 777's to AMS each day.
Capacity dumping will just lead to more cheap seats and turning into the Pot-Shuttle which KL aren't really interested in.
Lufthansa tend to be quite acutely seasonal at ABZ. And it shows with it reducing to 2x daily and being operated by the CityLine E190's. For next summer the A319 is scheduled for S14 and is probably adequate with the odd A320 showing up every now and again I imagine.
When the service first launched there was a 9.30ish departure ex ABZ (now 12.30) which could be a good time for them to add a fourth daily but can't see it happening in the near future but if so I suspect it will be around this time.