I've heard some claims that blending would break up the fruit in too small many pieces leading to sugars that absorb more quickly - but I've never seen anything to back these claims.

One difference is of course that when you eat the fruit, you chew it. This takes time and this may give some "food coming, hunger over" signal to the brain. Smoothies are so easy to drink that you might be getting way more fruits than you would normally eat.

What is the problem with rapid metabolism? Isn't that a good thing, so long as you have the varied nutrients to ensure the long tail of metabolism? E.g. A mixture of fructose, glucose, and more complex carbohydrates.