Why is web page print from iOS Safari garishly large?

I have a website, in whose CSS I have specified "@media print { body { font-size: 10pt } ... }"

This works fine when printing from laptop browsers (including latest Safari, Chrome, and Firefox, and for what it's worth, on an Epson printer, OSX 10.11.4...)

When I print from iOS (9.3.1) Safari) the rendered print is garishly large!

N.b., the printing is all done via Wi-fi.

N.b., the website is set at "max-width: 31em" ... and the iOS Safari fills the paper to width, while the laptop browsers respect the specified font-size and leave plenty whitespace on each side of the print block.

I don't find this behavior anywhere documented. Is there a way to avoid it? (Like, in CSS?)