I don't necessarily agree with that. There's being in Thor shape and there's being in Chris Hemsworth shape.
Normally and like CH did for Ragnarok he will bulk up immensely before filming and become absolutely huge, then he will start to gradually "trim down" reaching peak form for the beginning of the shoot and, since is so hard to maintain that physical stature, it doesn't last long until he starts to lose the weight, muscle and overall size. They even shuffle the schedule of the shoot in order to get the most "visually physical appealing" scenes first and then just try to maintain a good form as much as possible during the 3 months of the shoot.
There's plenty of evidence to support what i just said. From Taika, CH, the crew, his trainer... As i mentioned one example was Ragnarok and how his body changed from the early scenes in Muspelheim and some other scattered scenes throughout the movie to the scenes in the arena...