Size usually, bubblers also usually aren't as intricate and more portable (some bubblers have built-in bowl/stem for example)
A bubbler is generally a normal sized pipe with a water chamber, basicly a hybrid pipe between a normal pipe and a bong. these are some examples: http://www.grasscity.com/img/products/sherlock_bubbler_hurrican_1.jpg https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSLXzBGbvCmm2tnAthygdSR90ugSdrWntAj4WHbaKhFbbFN0N1U A bong is basicly an oversized pipe with water, characterized with a main shaft taht stretches upward, examples: https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSGVWQ6Ng6RwSh0snrfAJdQuTIrqoIeDa0LEi9AQ65vqnNTv_LY http://www.premiumbongs.com/images/roor-bong-001.jpg