I think it isn't even a matter of bigness per say but of monopoly essentially. Your local water company can't decide to just jack up prices to $2500/gallon because they have an exploitable monopoly position.
Meanwhile say GE could be very huge and produce 50% of lightbulbs but if they decided to be grossly unreasonable people could just stop buying lightbulbs from alternatives. Now if they had created their own bulb-socket standards and enforced them so that only they could produce bulbs that fit into the socket then there would definitely be an abuse of monopoly standard and ample grounds to argue "No you abused the patent and it will go into public domain now as part of the punishment.".
Meanwhile say GE could be very huge and produce 50% of lightbulbs but if they decided to be grossly unreasonable people could just stop buying lightbulbs from alternatives. Now if they had created their own bulb-socket standards and enforced them so that only they could produce bulbs that fit into the socket then there would definitely be an abuse of monopoly standard and ample grounds to argue "No you abused the patent and it will go into public domain now as part of the punishment.".