.......Hoping someone with authority on the subject can give a definitive answer.
Sorry, I'm not an authority on the subject, but I'll risk venturing a guess anyway.
My guess would be that the lines are different diameters for reasons of frequency. If you've ever used an ABS at its limit, you would have felt the hydraulic pressure pulsations feeding back thru the brake pedal. It seems logical that you would not want any natural frequencies in the hydrostatic brake circuit coupling with the operating frequency of the ABS valving. Such conditions would adversely affect the ability of the ABS to accurately meter the brake fluid discharge.
Just my uninformed speculation. So take it for what it's worth.
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