Sprue former should be attached to the bulkiest portion of wax because:
The sprue former should be attached to the bulkiest part of the wax pattern because of the following reasons:

This will minimize the effect of released residual stresses by the heat of attaching the sprue.
It will ensure that the thinner cross-section of the mold will be filled completely.
The melt will always be fluid enough and available until all lesser dimension sections are adequately filled.
Minimizes the effect of released residual stresses
To ensure that thinner cross section of the mold will be filled
Melt will be fluid enough for all lesser dimension sectors to be adequately filled
All of the above