I'm thinking it has nothing to do with the IFrame name (Although, you could try using the id property instead of the name property - this probably wont change much though)
Because you're using FTP, It's just loading IEs FTP interface, when you click on a folder or something, it'll open it in the full window because the toolbar needs to change to display all of the FTP control buttons.
I'm not sure that there's a way around this.
~Quack