Chip resistors are almost uniformly rated for 0.1W: 0402 to 0805, very roughly. The limiting factor is thermal diffusion, which has a sqrt(length) proportion: if the aspect ratio is not changed with size, then the total dissipation varies relatively slowly with size.
They also assume a naked chip resistor with thin traces; you can get much more from an 0603 with heavy copper pours around.
TH resistors vary, but temperature of the materials is probably the biggest factor. Epoxy vs. silicone enamel (or ceramic, or vitreous!), tinned vs. welded connections, etc. And whether they guarantee tempco or tolerance within the temp range, or what.
Tim