A shorter frame, of the same layup, out of the same mold will feel stiffer then a longer one.
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Yup! Stiffness is an extrinsic property. I’m not familiar enough with junior frames to know whether they’re just an adult racquet that’s been shaved down. If so, then it’ll play stiffer for sure. But If the beam dimensions are thinner, that might not hold true. If you’re worried the racquet is too stiff for your son, maybe take a more flexible adult racquet and try shaving down the handle to make it 26”. Like
@Howard H mentioned, I really don't see anything wrong with playing with a 27 inch racquet though. Back in the day, kids used wooden frames and those were quite heavy. If anything, using a heavier racquet will actually help your son develop better stroke mechanics. And contrary to what some ppl believe, heavier is better for injury prevention.