What limits a motherboard from supporting more memory?
I have a netbook (HP Mini 110) with limited RAM. It shipped with 1 GB and according to HP that's all it can handle. Yet I've read here that it's possible to upgrade at least some models of the series to 2GB.
This raises a question: How can one determine the true hardware- and/or BIOS-imposed limit on the amount of RAM, and what factors contribute to said limit? I'm specifically excluding OS-imposed limits, as such issues can be worked around for people who are willing to switch OSes as necessary. Please also bear in mind that I don't know too much about hardware; I'm a software guy.
EDIT: According to the comments, the motherboard is a potential limiting factor here. Why? What makes the motherboard (or some other component) consider a 2 GB stick to be OK, but not a 4 GB stick? Basically, I'm trying to understand the technical reasons behind memory limitations.
Info about my specific situation: I'd like to upgrade to 4 GB if possible, but I don't want to waste money. However, this question isn't really about my specific hardware; it's more general than that. While I'd be interested in hearing any personal experience with my particular hardware, I think that kind of stuff is best for the comments, as this question is the one in bold above.