Pair instability supernovae are thought to end the lives of stars with initial masses $>130 M_{\odot}$.
For SN2007bi, the relevant paper by Gal-Yam et al. (2010) deduced that they had seen the explosion of a $100M_{\odot}$ helium core and they infer that such a massive core would have arisen in a star with an initial mass of $\sim 200M_{\odot}$. So there is no contradiction here between theory and observation.
The core of a very massive star relies on radiation pressure as a means of support. Pair production removes gamma ray photons that were providing pressure support and replaces them with the rest masses of (relatively) slowly-moving electrons and positrons, which do not.
What matters here (for the pressure) is the kinetic energy density, not the total energy density - which is roughly constant. Pair production has the effect of turning the pure kinetic energy density of photons into the rest-mass of electrons and positrons, thus reducing the pressure.
Of course, the matter and anti-matter also annihilate giving back the photons, but it is an equilibrium process such that once a population of (albeit short-lived) matter/anti-matter pairs are created, they reduce the radiation pressure. The idea is that in the cores of particularly massive stars this is a runaway process, with core contraction leading to greater pair production, more contraction... And ultimately a supernova that blows up the whole star.