Sorry if this has been a question that has been asked before, I couldn't find it when I searched the forums. Speakers are supposed to have better "bass development" than headphones. I myself have noticed that they sound different. My question is, what is it that is physically different between the two? Are the waves more dispersed with better development?

I don't know what "bass development" is. But the bass should sound different for several reasons:

1) The room will significantly distort the frequency response of speakers in bass frequencies which could make the bass sound stronger or weaker depending on frequency and location in the room
2) Hearing sensitivity to low bass frequencies drops with frequency and the perception of bass is supplemented by feeling vibration through your skin. So a speaker can help you "feel" bass you may not experience the same way via headphones.

Depends a lot on the speakers and headphones. My Hifi system is 3dB down by 45Hz, my home theater is 3dB down at 25Hz and my earphones are 3dB down at around 30Hz. The headphone bass is less obvious though, mostly due to room effects (as DigitalMan said).

Along with the two effects that Digital Man mentioned (which are likely to dominate), there is at least one other reason that bass sounds different with speakers. This is because headphones do not have frequency dependent group delay, and speakers (especially Bass Reflex designs) do. There is a perceptual limit here, but above the limit the effects of group delay "disperse" the sound and make the bass hits sound different.

Speakers (and headphones) can have a very obvious effect on the way bass sounds, and there are huge differences between speakers. Powerful cheap subs are the worst - they exhibit high levels of dispersion due to the cabinet alignment, strange effects due to inadequate cabinet bracing, and chuffing from the port due to excessive port air velocity at high volumes.

Headphones, on the other hand, typically have a less flat response than good speakers, but tend to produce a much more neutral sound. This is mostly because they don't interact with the room, and produce much, much less sound energy than speakers.

Air is not dispersive at sound frequencies, for the most part.