No, it's limited from the factory and is not user-upgradable. It's long past time for Apple to retire the 16GB entry-level models and bump up the minimum to 32GB. It's also time to embrace the future and include 2-3GB of RAM for internal memory.
It doesn't need to. Too much ram is just too much ram Android only needs to run 512mb of ram and the rest is due to skins and excess manufacturer features. Also people have a misunderstanding on how Android management system works. People are falling for marketing and numbers if they believe all devices should have 3+GB of ram.
Also I would like to add ironically majority of people only need 16GB on any smartphone. Most people choose to have more if they can. I know of people who use up almost all 32GB of their devices and if they actually think about what they install and use regularly near 2/3 of what they have installed and not touched in 2 months would be available.