Well the large growth in number of Ukrainian Catholics is explicable by the fact that when the Soviet Union collapsed, all those repressed Ukrainian Catholics in Ukraine whose church’s very existence was illegal and punishable under the Soviet Union came out of the catacombs. Until then, if they wished to join a Church, it had to be the Soviet-approved Russian Orthodox Church. Many went to this church in the absence of their own liquidated church but secretly still being Ukrainian Catholic. Church attendance in the Soviet Union, as Bohdan Bociurkiw pointed out, was actually the highest in the Ukrainian Catholic homeland of Halychyna, with sometimes priest and parishioners knowing they were really Ukrainian Catholics. (Others risked their lives by attending clandestine Ukrainian Catholic services). To this day I think the Russian Orthodox Church finds it problematic that the highest per-person church attendance of all the lands of the former Soviet Union is found in that part of western Ukraine where the Ukrainian Catholic Church was and is - Lviv, Ivano-Frankivsk, Ternopil, and the Mukacevo eparchy in Zakarpattia (the latter province being the ancestral homeland of those faithful on CAF who put Ruthenian in the upper-right hand corner and which church also appears on the chart and which rose to life again after the collapse of the Soviet Union and unimaginable suffering).
It is truly miraculous that after decades of death, torture, and repression, millions upon millions never lost their faith in their church.