That's bothers me too. Perhaps Jiren's gimmick is he can collect power over time and store it for large bursts. Like he was storing his power before the tournament, unleashed it for Goku, then went back to meditating and storing again for the next fight. His average power may be lower, and he can just fight at these heightened levels for a while.
That's why the clown GoD told him "the time for preserving your power is over."
Maybe I'm reading too much into it and it's just poor writing/plot hole XD.