If I'm correct the warlock glide ability is based on the fact that light can alter reality (Physics) (To be exact the glide ability changes the effect of gravity on the user) If the traveler can do something similar it won't matter how heavy it is as long as it contains sufficient amount of light. This is probably why some of the citizens of the city are worried because if its light wavers it could crush the city or maybe even alter the orbit of earth if your calculations are correct.