I have a bunch of these capacitors in my collection:

1. Both are 0.1 µF.
2. The smaller ones are rated to ±10% tolerance.
   The K on the larger ones indicates ±10% tolerance
3. The smaller ones are rated to 50V.
   The larger ones don't specify voltage, but it seems unlikely they'd be rated less than 50V.
   (They'll be used in projects which utilize between 3 and 12 volts DC)
4. Both are ceramic unpolarized.
5. The larger capacitors are older but I read that ceramics are far less likely to fail from age than electrolytic.

Is there any reason I shouldn't use these interchangeably in projects?

Thanks in advance

 

They are both ceramic disc capacitors.
You can use them in most places that call for a 0.1 μF non-polarized disc capacitor.
Use the smaller one for power supply decoupling where the power supply voltage is less that 50 V.
Use the larger one for AC signal coupling, filter circuits, and for timing applications.

 

Thanks Mr. Chips!

 

The difference is size is likely either due to them using different ceramic dielectrics, or the smaller ones being multilayer and the larger one being single layer.