There are multiple things that contribute to your weight:
Body part | Increase | Decrease |
Base, without the extras | Higher Thickness
Being a taur, naga or drider
Being taller
Bigger hips
Having more legs | Having the Hollow Bones perk
Being a goo (i.e. having the Gooey leg type, lacking the Prehensile flag and having either a count of 1 or the Amorphous flag) |
Breasts | Breast size
Thickness greater than Tone | Having the Fertility perk |
Milk | Higher milk quantity
Certain fluid types weigh more than normal (honey, milksap, cumsap, nectar or blueberry yoghurt) | Having the Milky and/or Treated Milk perks
Certain fluid types weight less than normal (every type not listed to the left, except for cum, gabilani cum, nyrea cum and vanae cum) |
Belly | Being taller
Thickness greater than Tone
Being pregnant (live birth gives the greatest weight increase, then eggs, then seeds)
Any and all cumflations
Having ovipositor eggs | The Breed Hungry perk reduces the extra weight from being pregnant |
Butt | Being taller
Bigger butt
Thickness greater than Tone | |
Clitoris | Clit length
Clit count | Having the Hung perk |
Penis | Total cock colume
Having a knot
More cocks | Having the Hung perk |
Testicles | Ball size
More balls | Having the Bulgy perk |
Cum | Higher cum quantity
Having an ovipositor and the Nyrean Eggs status effect
Certain fluid types weigh more than normal (honey, milksap, cumsap, nectar or blueberry yoghurt) | Having the Potent, Breed Hungry and/or Bulgy perks
Certain fluid types weight less than normal (every type not listed to the left, except for cum, gabilani cum, nyrea cum and vanae cum) |
Also, if you checked the tooltip for the option to change your Thickness, you'd know that option requires taking at least five doses of GaloMax, but that would certainly make you a goo, which would be counter-productive to getting heavier (there are other ways to increase your Thickness).