Possible to shrink expanded O-rings

BTW, I tried soaking them in brake fluid for some days which made no difference. I wrote that they are tiny which in this case is approx 35 mm.

Since this is the brakes you are talking about, do you really want to take a chance that the rings are o.k.?
It looks like the oil has swollen the rings and has not exuded back out. You could try soaking the rings in a solvent in order to increase the mobility of the oil, but at the risk of overswelling the rings and having them rupture. Heptane, toluene, gasoline are all options. They are all flammable options as well, so keep it away from ignition sources, have good ventiliation,...
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I have considered that too, but there are two rings per brake piston. The inner one that keeps the brake fluid inside haven't been damaged at all while the outer swollen ones are for keeping dust outside. Because of that I think that the risk of a sudden failure is absolutely minimal. I wouldn't recommend other to experiment with this, though, and I only do it because it's my own bike and I feel confident that it won't failure suddenly.
I left the rings in gasoline over the night, about 6½ hour, and after 10 hours of drying they have shrunken to their correct size. When I removed them from the gasoline they had swollen to around the double circumference but they seem perfectly free of ruptures now (even when stretched a little). It seems that your tip has worked perfectly, so thank you very much! :-)