Tables can resize depending on whether you have table and/or column widths defined and the unit used to define any widths. (Tip: Only use % to specify widths.)
However, there is a certain minimum size based on the content/number of columns. So they won't resize past a certain point which will be somewhat different for each table.
For general information about styling tables and responsive tables:
Getting any of these working in Robohelp is a different kettle of fish, which I can't help you with I'm afraid. But hopefully the information is helpful.
I've noticed this too, and resolved it by not defining any table width (% or otherwise) in the HTML.
Note however, that there's another issue I'm having with tables, where they won't resize if they have any images in them i.e. the images themselves aren't resizing, and hence it looks like the tables aren't resizing either.