I know with alter self you can make yourself practically weigh nothing but i was curious if there was a limit to how heavy you could make you character. My theory is you could make yourself as heavy as you wanted but at a certain point you would end up collapsing under your own weight.
Some people might ask whether alter self can change your weight at all, given that the text says "change appearance" and not "change form". But note that this spell is a transmutation spell, not an illusion spell. The changes produces by transmutations are changes to your form.
Alter Self says: "You can make yourself appear as a member of another race, though none of your statistics change. You also can't appear as a creature of a different size than you."
So, for example, you could change yourself into a dragonborn, which has "average weight" 250lb. It's not clear in 5e what the upper bound is for a medium size creature.
A DM looking for guidelines on the upper bound for a medium size creature might look at the table for 3.5e, which says that a medium size creature usually weighs "between 60 and 500" pounds. Obviously this is just a guideline -- and a guideline from a different edition, at that. Ultimately you will have to let your DM decide.