For some reason, it's getting harder to gain weight as I get older

‌
Whenever there is a special occasion like a holiday and I go back to my hometown, I usually eat to my heart's content for about five days.
After eating like that and returning home, I check my weight and find that I've gained as much as 3kg.
However, after about 2 or 3 days, it returns to my original weight.
To maintain that increased weight, I would have to continue binge-eating like I did during the holidays,
but as I get older, my food intake has definitely decreased. As soon as I feel like I've eaten a bit, I feel full.
Because of this, my weight hardly changes throughout the year.
Some might say, "Isn't that a good thing?" but there is one major downside to looking thin when you enter your early 40s.
If you look thin, it usually shows in your face, and since collagen decreases with age, your cheeks hollow out, making you look haggard.
Even when I'm not particularly tired, the area under my eyes hollows out and my dark circles become prominent. If someone saw me, they might ask if I'm sick.
When my weight was over 75kg, I didn't hear those comments, but since I've dropped to the low 70kg range, I hear them much more often.
I am 178cm tall, and while a standard weight would be around 70kg, it seems my weight makes me look thin rather than just standard.
I think a weight of around height minus 100 might be the most aesthetically pleasing weight.