"If I were lighter, I could fly faster" is the correct answer.
When creating the comparative form of an adjective, all short, 1-2 syllable adjectives simply add an "er" to the end:
longer, faster, greater, poorer, tighter, etc.
Some of those types of adjectives have doubled consonants:
bigger, taller, hotter, etc.
For longer adjectives with 3 syllables or more, you use "more + (adjective)"
more interesting, more difficult, more peaceful, etc.
There are exceptions though, so you just have to remember them:
more fun, good > better, far > farther, etc.
This is just a quick description of some of the rules for adjectives in the comparative form. There are more details to be known, so have a look at this link if you would like to learn it:
I hope this helps! Good luck in your studies!