Why is Cl a better leaving group than an anhydride?

[nobahar]

Hello!
I just read that chloride is a better leaving group than an anhydride (when adjacent to a carbonyl, hence the anhydride). (i.e. R-COCl, an Acyl chloride and R-COOCO-R', an anhydride).
I do not understand why this is so. A good leaving group is characterized by being electron withdrawing and a stable anion. Oxygen is more electronegative and a carboxyl group has resonance, which chloride doesn't.
I'd then conclude that the anhydride is a better leaving group; but apparently not!
Any help appreciated.

 

[cep]

Instead of just thinking about the leaving group as a discrete entity, think about the molecule as a whole, and about what interactions can be taking place between the LG and the reaction center.