Sketching on iPhone 6S+ using Notes - res problem

I have been doing small sketches on my 6S+, and exporting them to my iMac for organizing into a small book using InDesign. I usually export as a PNG using iMessage, and open as PNG, and they flow right into InDesign. They had great resolution before -- max dimensions were 1536 × 2048, around 2mb, big enough to look good on a half page layout. But now suddenly the export as JPEGs, at less than half the resolution -- dimensions are the same, but mb is 512k. Does anybody know why this has happened? Is there a way to max the resolution in Notes?

Or is there a good, simple alternative app that I should be using?

Thanks!!