Is it normal for allergies to affect one eye and cause that eyelid to be bigger than other. I’m worried it’s ptosis or something serious. I’m 19.

No . allergies usually involve BOTH eyes!! However eyelid swelling is much more commonly associated with BLEPHARITIS ( an infection of the lid) and local antibiotics and soaking (using an "eyecup") may be indicated. (This is a LOT more common than PTOSIS!)
Hope this is helpful!
Dr Z.