Just because one Volvo model used a certain bar size doesn't mean another model should. There are many variables in a suspension system such as where the bar is attached and what mechanical advantage it has to the wheel that can greatly alter the effective bar size.

In the later 960,s the stock front bar is the same diameter as the earlier 960 but the attachment is directly to the strut instead of the control arm so its effect is greater.

The rear bar on the Multi-link II system with the composit transvers leaf is of 3 stock sizes depending on the car/wagon/Nivomat species. Its attachment points and mounting are nothing like the front so the effect can't be compared.

For a given bar shape, the stifness increases as the square of the diameter as I remember.
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'96 965, 16' wheels, Michelin Pilot Sports, rear 18mm bar + Koni, 201 HP cams, 134K. Put 200K on '85 745 TD.