It seems to me that they would be adjusted as armor would, but I can't find anything verifying that information.

Weight: This column gives the weight of the armor sized for a Medium wearer. Armor fitted for Small characters weighs half as much, and armor for Large characters weighs twice as much.

I'm not as concerned about the price right now since changing from Medium to Small doesn't change price anyways, but if you find information on whether that changes for pairs outside of the usual size range (such as a Huge mount or a Tiny rider) it would be useful to know as well.

• The only place that I know this is addressed—and even then only obliquely—is in a D&D 3.5 sourcebook. Would that be acceptable? – Hey I Can Chan Aug 5 '19 at 8:42
• Well it's better than nothing at this point. I've done a lot of searching myself and haven't found anything. – Dorian Aug 5 '19 at 9:08

$$\\begin{array}{|l|l|l|} \hline \textbf{Mount Size} & \textbf{Saddle Price} & \textbf{Saddle Weight} \\ \hline \text{Small or littler} & ×0.5 & ×0.1 \\ \text{Medium} & ×1 & ×0.5 \\ \text{Large} & ×1 & ×1 \\ \text{Huge} & ×2 & ×2 \\ \text{Gargantuan} & ×4 & ×5 \\ \text{Colossal} & ×8 & ×8 \\ \hline \end{array} \$$