Is it normal for petite women to feel like they can't defend themselve

Gypsy, once again.

Ignore that, you really do not "need" anything, such as a larger person, to protect yourself. You may not want to become "bulky' or large in muscle mass, for your frame. Being strong is more than physical, it's mental as well; I had a friend who is quite small, take on a larger guy who was with friends trying to harass our group of friends, outside of a bar, although I do not condone violence, I jumped in and had him on his back - granted I work out incessantly, as a personal trainer/strength trainer, I definitely had no problem defending "us", as two primary females in a very crude situation. The guy was trying to push her (she is 5'5 maybe, 100 or so pds) and I'm 5'7, about 115pds in off season and 130/ in 'bulking' season, for shoots and such. Being incredibly strong from past CrossFit training and my athletic past, has helped my body maintain strength I didn't know I had, at times.

Although I refrain from self-defense classes, I desperately wanted to protect myself and mostly my smaller friend (and an innate instinct came out) He was tall and built, but could be brought to the ground from sheer force, as I was incredibly angry/adrenaline spiked. I can't say I want to 'test' how many guys I could handle, but I knew in that moment, fight or flight, I fought and conquered. As a tall/built girl, it was no problem. But she could have been punched, or worse, as this guy was ridiculous, in trying to over-power her.

The pepper spray (to all of us) in the situation at hand - security rushed to detain us - was a problem. It wasn't pleasant, but everytime I workout, I remember that night.. I'm strong and confident that I can defend myself the best I can, through weights, tactful reaction and understanding your "size" is not the issue, it's overcoming a standard you set for yourself; "If you can lay on your back and be beaten, you can put up your fist and fight" -- you can try anyway. One of my favorite notes.

Try to lift or build your strength, both physical/mentally; if you feel you're worried you may need such strength if in a worrisome situation, one day. You do not have to let your size dictate your abilities. I know short girls who really impress me (some ex-gymnasts, some crossfit competitors, some "fighters") and they are the epitome of well rounded and confident when walking/jogging in public.

It's the fear that can immobilize you.

Start lifting and bulk up. You could put a solid 40 pounds on your back in a couple years. Then you can simply throw your enemies across the room.

You're the same op as the guy that has a "fetish" for weak women, huh?

As a tiny person, of course you are afraid large people. There is no way you are going to overpower a large person intent on hurting you. So you are going to have to use your brain and a few tools. A spray or tazer is great if you want to let someone that close. There is another electric charge gun the shoots darts with a huge charge, which hits them farther away, I don't remember the name. You could always get a firearm, but without training, AND the will to actually use it, then that's just plain stupid. You are too small to point a gun you won't use, at anybody who has evil intent.
Vomiting on your attacker is a very effective method of giving yourself time to get away, but you'd better have an exit plan in place. Best plan of all is to have a 6'6" boy friend who will walk with you through life, and see that you don't come to any harm.