Many reasons exists for different sizes.
To start there are legitimate reasons, as listed below, to have different sizes where some of the specs you list also differ (many of these apply to L, C and R, some a bit more to one than the others):
- Different Power Handling Capability
- Different maximum voltage
- An option on the larger one to later add some laser trimming option for customised values (manufacturer looks to the future?) post production [not the most likely to be honest]
- Different Parasitic Capacitance
- Different Parasitic Inductance
- Different Peak Power (even if average power is the same, peak power may be higher or lower in different packages).
- Able to use a different ceramic with other storage characteristics
- Allows for different processing techniques in the factory
- A large package is easier to pick and place, so for an 0805 it's easier to find a cheap PCB-A manufacturer than for an 0402.
- Grouping and mismatch is easier to manage in small packages for high frequency.
- Different Reflow profiles for different size components, due to the use of different coatings or end caps (this is far fetched for common components, but does happen in specialised areas).
All these reasons mean there then are different sizes of packages. Assuming there already are different sizes of resistors (for example) any manufacturer worth anything will make their resistors in all those sizes, so a designer can switch to them for all their resistors. Makes sense right?
So then, if you make a design for a nice 0402 resistor of a value using a machine for that design type, which also happens to fit a 0603 ceramic package and maybe even an 0805 package and market research shows that in that value range (for which 100k certainly seems to qualify) hardly anyone needs more than the power capability of an 0402 and all other things are nearly equal for the larger ones too... then why not implement one design/machine on three bases, cut a tiny bit of cost? Even though the larger one possibly can handle a bit more power due to its larger bulk, the ceramic may be so cheap it doesn't conduct well enough to make good use of the extra surface area, so you specify everything pessimistic, which is same power as 0402 and same parasitics as the largest package.
Obviously, a high end manufacturer, in their high end range, usually tries to get the most out of every size as a point of quality and pride, but almost every manufacturer also has a B-range, where they optimise cost away as much as possible without making the product (much) worse. Some sell it as a different series, some sell it under a different brand name.