If you really must use them, make them at least 80% width and height and change the opacity of the black overlay from 0.81 to 0.6. Have the purple header snap to the top, and have it slim down to something like 20px height and hide the title when you are scrolled down.

http://snag.gy/LKED2.jpg vs http://snag.gy/ROfRO.jpg

Also, test those things on mobile. Modals are tricky on mobile.