[TF-Viewer]

Hmm..

Well, as many have said already her large chest is probably the first thing people tend to notice. Keeping on the subject of her appearance there are other things to consider as well. Such as:

The short skirt.
Long legs.
Long dark hair with a simple style.
Her gloves.

The fact that her outfit and hairstyle are fairly simple makes it relatively easy for someone to cosplay as her. Also her combat style and knuckle weapons means that one can complete the entire outfit including her weapons on a very small budget indeed. In contrast many other characters in the FF universe have large or complicated looking weapons, or weapons that are both large and complicated. Tifa can get by with just fingerless leather gloves.

The simple style also makes fan art of her easier to produce as well. Not having to draw a hundred straps, buckles or belts, nor any ornate hair accessories is probably a relief to artists.

I think even if her chest were a few sizes smaller she'd still be fairly popular based solely on the simplicity of her design. Being a main character in the most popular FF installment doesn't hurt either I suppose. Or maybe it's just the big breasted and easy (to cosplay and draw) aspect that's responsible for her fame.