step 1: open a graphic editor with layers (there's web-based pixlr.com if you don't want to install anything)
step 2: get your comparison pic into the program. use "open image" or "open image URL" depending on whether the photo is saved to your comp or not. alternatively, create a new image (file - new image), click printscreen (only this will work with pics from afriska.ch) and click edit-paste.
step 3: on the right, you'll see the layer titled background. right click and choose duplicate.
step 4: click the toggle layer settings button, bottom left of the "layers" window. change the opacity and move the layer! you'll see one cup kinda on top of the other one.
if this sounds complicated, i recommend using the pics from afriska.ch as they're very clear :) otherwise you can use free transformation on practically any pic (transform the half-transparent layer). Here's an example, I used a pic taken by femininewear :)
i marked with blue the part where i *think* my old medium meluna is too narrow for me and where i *think* the new small's different shape will help :) let's see whether that's really so...
also, I've recently experienced almost the same as what's described here, well not quite ;D I got some geisha balls and despite having a really low cervix, i can insert them when my cup is in! XD they're fairly large, 3cm in diameter, so yes i'm impressed there's space for them haha.