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Probably because the pipe from the turbo to the intercooler is made of thick rubber to withstand the heat from the turbo. It's also a pretty straight hose so there probably isn't much to gain since it can't really be any thicker in diameter than it already is.
Now the pipe from the intercooler to the throttle body is made of some sort of molded/glued plastic material. It has bends and seams which are prone to fail over time with heat cycle expansion and contraction plus additional boost pressure.
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