The smaller numbers are thinner walls and lighter weight. The frames will flex more and teh bikes will be lighter. Because of the thinner walls, the tubes are less strong than thicker ones. There are a lot of dimensions to strength of bike tubes and how significant it is, but what this mainly means is that if you put an 017 tube into a machine designed to pull tubes hard enough to stretch them and finally break them, the 017 would stretch more and break at lower tension than the 022, or other thicker-wall tube of the same steel. In the real world it's really hard to say what that will mean. Notice Ishi says the 017 is good for Record Attempts. Those will be strong riders, but I think the frame will be able to handle them. For us normal riders, I think any of them should be ok. Being more flexible, the thinner tubes could result in a frame that feels too flexible or noodly.