I'm late to the party, seriously late. Many meer smokers have spent years getting their pipes fully colored. It's what a good meer should do, soak up residues and color from the bottom up. The very nature of the mineral means it will usually be splotchy in color. And, yes a well colored meer will be noticeably heaver than a pure, brilliant white, unsmoked meer.
Depending on the "grade" of the meerschaum, some bowls will be heavier than others, not as pure in color, etc. and all will gain in weight as they color. The voids are filling with tars, oils and other residues from the tobaccos, toppings, casings, including whatever is in the air you are drawing into the bowl as you smoke. It's the nature of the mineral.