Image reduction when posting to Pinterest

[pentaxaf]

I am using a Verizon Galaxy S7, and previously had an S4. On both phones, when posting to Pinterest from the phone's image Gallery, stored hi-res pictures (say, 1600×1200 px) reduce automatically and substantially (say, to 780×500 px). I don't think it's a Pinterest issue, because hi-res pix otherwise smoothly post full size. By contrast, in email sharing, you can choose the size or reduction you prefer. Am I missing a setting, or does anyone have any advice how to fix this?