Usually it is said that small SMD capacitors (e.g. 0603) have lower ESL than larger ones (e.g. 1206). This older question addresses the same problem and the answers confirm what I was thinking, namely that the ESL is determined by the current loop through the capacitor for simple MLCCs.
Now consider the following case of decoupling caps that connect to a power/ground plane pair:
Which capacitor would have lower ESL and why?
Wouldn't the 1206 have lower ESL precisely due to its smaller current loop? Isn't the "capacitor ESL" actually a misnomer because the ESL of the decoupling network is critically caused by its connection to the power rails, i.e. by the number, placement and spacing of the vias rather than the dimension of the capacitor?
The following graphic also seems to suggest that the current loop can be greatly shrunk by placing the vias in tight proximity, because the majority of the current loop is between the vias:
While those packages all have roughly the same size, inductance differs by more than an order of magnitude. It suggest that the current loop area is the only (or by far dominant) contribution to the package inductance. Of course, reducing the package dimensions facilitates achieving smaller total loop areas. However, if this is all there is to it, why do all those guides on decoupling talk about capacitor size so prominently, when it is only a small piece of the whole picture, namely the effort to minimize current loop area? What is the physical reason for MLCC ESL? Is it indeed only the current loop inductance or is there more to it, e.g. some permeability of the capacitor material which of course matters less, the fewer capacitor material there is.