Double-sided Assembly
You can find some good and relatively modern information in this document. 

WEIGHT LIMITS FOR DOUBLE SIDED REFLOW OF QFNS
Sasha Smith, David Connell and Bev Christian

Although their tests were for QFN packages, they work with the ratio of total pad wetted area to package mass. It will vary a bit with the solder type as well, in the paper SAC305 (96.5% tin, 3% silver, and 0.5% copper) is used. 

They also refer to an older "rule of thumb" formula in unpleasant mixed units:  

\$\frac{\text{Weight of the component (grams)}}{\text{Sum of the area of all solder joints (square inches)}}  \lt 30\$

Of course you can always glue the parts. It's often possible (and often desirable) to keep all the heavy parts on the "top" and lighter parts on the bottom.