Electronic – 0805 LED on a 0805 resistor footprint

The current standard is IPC-7351B, which replaced IPC-7351A, IPC-7351 and IPC-SM-782 (in that order). Mentor Graphics have a free PCB land pattern viewer for Windows (old link; they rebranded to PADS) for all of the standard parts using this standard. Each part also includes a "courtyard" layer which defines how much space needs to be left around the component for manufacturing; useful when you are designing high density boards.

The standard itself consists of three versions but for most boards it is recommended that the "N" suffix (Nominal) footprints are used. Note that most parts do differ slightly from manufacturer recommended patterns but in my experience (and the experiences of my board loader these footprints do tend to meet manufacturing requirements very well.

As for you query about the 0805 resistor vs 0805 capacitor, I suspect that the footprints have been designed to best attach the parts; while they are fairly similar in the horizontal plane, capacitors do tend to be slightly higher so maybe the differences in the footprints take this into account.

Power rating on resistors is the primary difference but also maximum voltage rating can be lower on smaller packages. For capacitors, the maximum voltage rating will be lower on smaller packages and it is likely that the dielectric type will be "worse" for the same value in smaller packages. A "worse" dielectric means higher losses and/or capacitance drift with temperature.

For inductors, the effective series resistance will likely be higher on smaller packages but this can be traded-off by the inductor supplier using a higher permeability magnetic material - the downside being that it won't be very good in higher frequency applications. Also, smaller package inductors will, for the same inductance value as a bigger package, use thinner wires and thinner insulation on the wires and this can lead to lower voltage capability and lower self-resonant frequency due to the winding capacitance being higher.