Tuning line width and flow compensation in Cura
That is very bizarre.  Since GCODE describe each movement of the printing head (so, the printer does not get to decide anything in terms of printing strategy, it just executes), I can only see three possibilities that would explain what's going on.

The print is being scaled up at printer level.  This could for example be due to your firmware having your steppers improperly calibrated and moving them too much for a given unit of measure (say you say 1mm, they move 1.5mm instead).  This is easy to check: if this is the case, your cube will be scaled up (so - using the example above - if your cube is 10x10x10 it will come out 15x15x15).

You are printing with a raft.  Then there is no problem with your set-up, the first layer(s) of a raft are not solid, but intentionally "grated".  Check your settings to verify.

Cura is producing the "wrong" gcode.  This could be really wrong (as in "you found a bug", in which case you should report it on their github), or just look wrong (as in "you found a weird combination of setting producing that gcode", in which case you should reset the settings to their default and see the problem disappear).  Either way, if the gcode is "wrong", you should notice the gaps in the gcode preview mode in Cura.