As for a single anecdotal data point on OGS’ rating volatility compared to EGF’s rating volatility, these are my rating graphs from both over a selected period covering a similar number of games (~140), albeit that the corresponding time period on OGS is about 2 years, while the corresponding time period on EGF is about 7 years.
OGS, rating spread about 1.5 ranks:
EGF (red line), rating spread about 0.6 ranks:
Ofcourse there may be other factors that might affect rating volatility. My EGF games are IRL tournament games. Perhaps I perform more consistently in that setting. I may play less seriously in my internet games. On the other hand, my OGS games are mostly correspondence and I do take those fairly seriously too.