That's actually a weird question.
You do know that GUI is automatically converted to JASS when the game starts, right?
So in the end, the only script that warcraft can read is JASS.
When GUI is converted, the script is massive. At least a lot bigger than what it could be when done in JASS from the beginning.
So if you just want a low file size, then you have to code in JASS.
However, Bribe is correct: importing files because they look good (instead of because they fit your map) is a negative point, I can even subtract points for that.
That wasn't really your question, but it does have to do with it ^^