This is why. The myosin filaments (thick filaments / A band) don't actually get smaller. They simply overlap with the actin filaments (thin filaments / I band). Since we're seeing this through an electron microscope, we'll be seeing a much smaller region with ONLY actin filaments, meaning the I band will get smaller while the A band stays the same size. Here's a picture to explain what I mean.
Thanks a million. Muscle contraction is MUCH clearer now.
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