If a mind mapper is wasting too much space and an outliner is too restricted, try TreeSheets.
In a nutshell, TreeSheets is a mix between outliner, mind mapper and spread sheet. The documents use a nested structure of cells (like a spreadsheet where you can create work sheets inside of cells). Using this nesting, you can easily create outlines with multiple columns.
Using different rendering styles, you can recreate something very similar to a mind map, too.
And it needs only a little memory.