Hi Melanie! I haven't hit my thirties yet, but I've done lots of research in the field of weight gain. Here's some of what I've found:
1. Genetics matter. Some people are simply gonna be bigger than others as they age.
2. Metabolism slows. Your metabolism processes calories, but it slows as you age, meaning less calories are consumed and it's easier to gain a little weight.
3. Your body could still be growing. Maybe not in height, but some people continue to develop into their thirties. Men can see hair sprout on their back in their thirties that was never present before.
4. Eating habits. As we deal with the stress of being adults with jobs and possibly kids, it becomes easier to lead a sedentary lifestyle and ignore what we consume, leading to an excess of calories.
Hope these help!