21700 are slightly bigger diameter, but because the batteries are rolled spiral-like from the inside out, bigger diameters have a disproportionately large effect on capacity. Large capacity 18650 cells tend to be around 3.5AH, and large capacity 21700 cells tend to be around 5Ah.

What does that mean in real life? Let's build a hypothetical battery.

These batteries all have the same voltage curve, so in both cases we'll put 12 in series for \~44v nominal. voltage. (\~ 50V fully charged, 36V discharged if you're nice to it)

18650 : We'll use, say, Samsung 35E batteries. If you're making it the simplest shape, you have 12 batteries one way, and let's say 7 in parallel the other way.

21700 : Samsung 50E, again 12 in series, but we'll put 6 in parallel to make the battery about the same width as the other one.

So the battery made with 18650s is 84 cells, 216mm x 126mm with cell weight of about 4.2 kilograms. Total capacity of 24.5Ah.

The battery made with 21700s is 72 cells, 252mm x 126mm with a cell weight of 5 kilograms. Total capacity 30Ah.

So not so much difference as you'd notice. HOWEVER, bigger cells are cheaper for a given capacity. At current small-wholesale rates that 21700 battery with the higher capacity it almost fifty dollars cheaper to buy the cells for than the smaller capacity one made of 18650s. It's also cheaper to assemble, because there are fewer parts, and fewer spot welds. There are also a lot fewer crappy generic 21700s on the market than crappy generic 18650s, so your odds of getting good cells are probably better. Since individual cells' capacity diverging over time is the ultimate cause of most batteries deaths, fewer cells in a battery is, on balance, a good thing from the longevity perspective.

So in an ebike application the 21700 pack may well come cheaper, is somewhat likely to be made of better cells, and may well last longer. But none of those a LOT, so I wouldn't let it determine your buying decision.