Dump loading chip resistors and heat dissipation
First: A rise in temperature of 100K seems quite a bit for a 1210 resistor with 180mW net load. Did you measured the load or is it just calculated? Is the resistor packed in an additional package (housing or such) or is it operated at free air?

Second: The resistor has three ways to lose his dissipated energy:

1) conducted over mounted board - which for his parts loses the energy over 2) and 3)

2) Air convection

3) Radiation

It is not that easy to determine how hot which surface gets under given circumstances. For a simple rule of thumb obviously we can say: Make the part bigger if the part is getting to hot.