PCB pads different diameter on top and bottom
What they are trying to say is that solder may wick up through the (plate through) hole between the bottom pad (where the solder wave is) and top pad.

This is exacerbated by the warming of the hole via the heat absorbed by the top pad.

Remember that there is a pre-heat portion of the solder cycle, and the larger the pad the more it will heat up, and the more heat will conducted from it to the hole plating.  The larger top pad will also serve as a heat source helping to maintain the warmer hole plating temperature.

When the solder wave arrives, it will flow father up the hole because the higher temperature improves capillary action of the molten solder.

Whether this is good or bad is a question.  If the hole fills with solder, then the conductivity of the trace will increase, which could be goodness.  OTOH, will this extra solder cause thermal problems?  

Will the insulation around the hole in the middle layers handle the extended heat of the solder plug, and increased expansion/contraction of the plug as the board heats and cools during operation?

(Plus there is a slight reduction in the amount of plating and solder used  if you cut the size of the top pad.  Don't know if the savings will show ;-).