From what I've read, earbuds have less (overall) bass response than over the ear headphones, due to the smaller (tiny!) drivers in IEMs. I have the general impression that earbuds tend to sound brighter, but I have no real evidence of that; it's just what I've heard with a few earbuds.
The main difference between full-sized headphones and earbuds is the bass response. There's no replacement for displacement (with dynamic drivers) and tiny drivers have a very hard time reproducing deep bass. Those westone's claim they're more or less flat to 10Hz, which I find a little hard to believe, but if they do it, they do it through pressure mode base reproduction.
In general, if earbuds can form a seal with your ear, then they can "fake" deep bass to a certain degree by using pressure mode bass reproduction instead of free radiation. The science is complicated, but essentially you can generate bass more readily in a pressure sealed environment than when radiating in the open air, so if you get a good seal in your ear they can generate lower bass than a driver that small otherwise could, but: 1) relying on pressure mode for bass sounds kind of muddy to me (because it tends to be very resonant with poor spectral decay), 2) pressurizing your ear canal to generate bass can cause pretty intense listener fatigue IME, and 3) the effectiveness is entirely dependent on how well the buds seal to your ears (if the seal is poor, so is the bass response).
The same criticisms can be applied to some close backed over the ear headphone designs as well, but they have better bass response in general because they use larger drivers, and not all closed back headphones are entirely sealed. I've even seen some closed back designs that include an adjustable aperture that functions as a bass reflex port, which is a neat feature, but recognize that you're essentially turning your ear canal into a speaker cabinet.
My own biases should be obvious from the above narrative
That said, I still use earbuds or close-backed headphones whenever I need noise isolation (when monitoring live playback, on a bus, at the airport, etc.), but I only use open backed headphones for home or office listening. I find I can listen longer with less fatigue. I will concede that I'm unusually prone to ear fatigue, I theorize from too much contact with heavy machinery and/or too many concerts without ear protection as a youth. I tend to listen at a lower volume than all of my friends, etc.