So after writing a thank you to my wonderful donors, I realized that my thank you letter only took up one side of a blank page with a tiny bit going on the other side. Is it going to look weird that I did not fill up both sides as if I am really lazy and do not want to say anything?
Sorry if this is a stupid question
Don’t worry about it. A full page+ is plenty of gratitude. 
In all honesty, most thank you letters are only a page, on the front! Most formal letters look a little odd when double-sided. You’re perfectly fine. Congratulations on the scholarship, and good on you for remembering to write a thank you letter!