The reduced flow allows air into the setup due to the siphon.... you need to resrtrict the the discharge line to allow a lower volume siphon. However, in doing so you will also create more of a likliehood of a blockage causing a flood.
Better setup... a siphon with an open channel/emergency. You can valve the siphon back to get things in balance just like the full size overflow.
Other options would be to put a P trap just before the sump and an air vent. You could also use a pile of rubble or a bucket to catch the bubbles...
However, my opinion is that high flow sumps work better so I would not reduce the flow
