Thank you for the prompt answer as usual.
Just to let you know, in the meantime I tried other contact form plugins and they display correctly in all the mobile devices I can try.
Would the Contact Form 7 code be at fault instead of your code then? I mean, if the Celestial code behaved like that with every form then I would say your explanation makes perfect sense but Celestial displays correctly on retina display with other plugins. What do you think?