Guide to Losing and Gaining Weight (Singleplayer)
Even if you pass the tiniest amount of time to see a weight change take place (they only occur after screen transitions) the sheer amount of time will cause you to lose weight despite filling up on food. I've heard this is due to the framerate being higher on PC.
Buy chocolate and eat about 9 bars, or enough to hear the weight gain sound effect twice. (Like a gurgle noise upon eating food)
Then immediately save and reload. No time will pass but the game will recalculate your weight.
Do not sleep. Just save and reload and your weight will increase.
Do not pay attention to the weight gain and weight loss arrow (it's kinda buggy). Instead to track you progress, navigate to the "Player" tab from the pause screen. Here you can look at the stamina penalty/health bonus number. They increase and decrease in increments of 0.75%, up to a 7.5% total bonus.
When you reach 7.5% health regeneration bonus, you have reached the maximum weight. The status effect 'overweight' will also be highlighted in red on the previous screen when you reach max weight.
If you do not want to buy chocolate bars in groups of 3, get the provisions satchel and you can hold 99 of each food item. This however is not required.
You do not have to camp, kneel, sleep, cook, eat camp stew, eat 3 times a day or travel.
Just eat, save and load and you'll be good. There is a lot of stupid advice online about gaining weight, with my method you can gain it all in like 10 minutes and for only a couple hundred bucks.
This is much easier, and much less expensive.
Simply pass time by sleeping the maximum amount and your weight will plummet.
A difficulty of this is that you can only sleep twice in a row before the game prevents you from sleeping anymore.
Simply save and re load and the game will remove this restriction flag. You can sleep heaps when you keep reloading the game. Occasionally it takes 2 sleeps to drop down another 0.75% but don't worry, it'll happen.
You will know that you have reached the minimum weight when your Stamina Regen bonus on the player screen reads at 7.5%.
Growing Hair (Bonus)
The method for this relies on a similar method to losing weight.
Save and Load to circumvent the sleep restriction after drinking a hair tonic and your hair will sprout incredibly quick.
Get the item satchel if you want to store heaps of hair tonic, this way you can hang out next to your bed and keep sleeping/saving/reloading to get the quickest results.
After you get to length level 7 for beards, the hair growth takes considerably longer between each stage and requires hair tonic.
I've read it takes over 100 in game days to reach level 9 or 10 length for the facial hair. I don't know if that's what you want, but if you do, you got a huge slog ahead of you. Could take 4 or 5 sleeps and therefore multiple saves and loads to go up a length. Be warned this will take considerable time.
Below level 7 length, hair will grow super quick, even without hair tonic.
Note head hair only goes up to length 7.