Why does sp2 hybridization result in a greater chemical shift in carbon NMR?

Carbon NMR has a very wide chemical shift range. This is attributed to the relative proximity of electron-withdrawing groups (1-2 bonds separation) compared to proton NMR (2-3 bonds separation) along with the hybridization state of the carbon atom (plus other factors such as diamagnetic anisotropy). The reason for the chemical shift difference between proton and carbon NMR due to electron-withdrawing groups is conceptually easy; however, I am having a difficult time understanding why sp2 hybridization results in a greater chemical shift. Hybridization results in smaller, more compact orbitals (closer to the carbon atom). This is what throws me off. I would expect greater electron density near the carbon atom due to sp2 hybridization to result in a shift upfield (not downfield). Is there any simple explanation for this?

• Because sp2 hybridized carbons have $p$ orbitals that form $\pi$ systems. $\pi$ systems create electronic systems that increase effective shielding. Your statement isn't wrong, but it's just not the dominant factor.
– Zhe
May 20 '20 at 1:53
• For 13C vs 1H chemical shift ranges see: chemistry.stackexchange.com/q/62546/16683 (also chemistry.stackexchange.com/q/81127/16683), it’s not actually what you said. Non-specialised textbooks (especially those targeted at organic chemists) often don’t tell the truth about NMR. May 20 '20 at 3:20