He is NOT the same size. Firstly old hulk is always slouching and he is still taller than the new one. Rewatch the end of ragnarok. Hulk is taller than Korg at the end of the film when Thor walks up to the "throne" on the ship and he is surrounded by everyone. Not by much but enough to be noticeable. Hulk is clearly the biggest there. Now compare that to when Bruce visits fat thor after the time skip. When they are talking and bruce mentions thanos. Korg walks up to them he is standing taller than Bruce. Its actually quite awkward. He is clearly meant to be smaller than regular hulk. In frame and muscle mass in general (especially the traps area). Hell there are multiple examples of this. In fact use that same scene of Smart hulk talking to fat thor in his living room. He honestly isnt standing that much taller than thor. Now go and rewatch the gladiator fight between the two and note the huge height difference between them. And thats a hulk that was also always slouching and would be even taller if standing straight up.
This scene was slightly different in the film but still a good reference. And here he still isn't standing completely straight.
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Compare to this
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Reminder thor is taller than cap and tony.
Smart hulk is like regular hulk's little brother.