Good question!
There’s a couple ways that that could happen:
- Marrying the neighbors. Dynastic alliances and marriages based on landed dowries wasn’t just for the nobility; very frequently, peasants would seek strategic marriages for their children with their neighbors, which would allow the children to merge the two holdings into one larger farm with better economies of scale.
- Cultivating new land…sort of. It’s not that you could just clear forested land or drain fenland by yourself and get a title on the land via homesteading, because that land probably existed within someone’s demense. More commonly, what you’d have happen is lords or churchmen looking to increase the profitability of their fiefdoms by paying people (often younger sons recruited from either nearby areas or foreign lands) to clear or drain the land in part by offering them tenures on the new land, with usually some sort of tax or rent holiday as additional incentives.
- Redistribution on failure of succession. While people of all social classes were incredibly defensive of their succession rights and would react badly to anyone trying to redistribute land that they felt was their inheritance, there were circumstances in which land tenures would become vacant. For example, if someone died without an heir or were convicted of a felony, under the law of escheat, the tenure reverted back to the lord and could be redistributed. If the heir could not pay their feudal relief (a one-time tax paid by the heir upon inheriting the tenure), the land would also revert back and could be redistributed. Etc.
- Bribing the lord for a land grant. Depending on the size of the lord’s land vis-a-vis the local agricultural labor market, it might be in the interest of a lord to lease out some of the land they held personally, if for example they didn’t have enough serfs to perform the labor for free or there weren’t enough workers in the area to work for wages, relative to the size of the lord’s personal land.