As Jmb2341 suggests, this over extrusion could be due to an excessive feed rate.
However, I would also like to add the possibility of your Z-height during the first layer being too close. For instance, if your first layer height is set to 0.4mm, but your print head actually moves as close as 0.2mm from the bed, there will be twice as much filament extruded than what it is actually room for below the nozzle. This, in turn, will appear like over extrusion, while the issue actually is your Z-leveling/height.
In general, unless you are using a raft, the amount of first layer over extrusion will often have to be balanced with the prints ability to stick to the bed. In other words, over extrusion during the first layer improves print stickiness, so that close bed leveling gives sticky prints but first layer over extrusion; while, distant bed leveling gives loose prints with more accurate first layer extrusion.
If you are wondering whether what you are seeing is over extrusion due to excessive feed rate or too close bed levelling, then you should try to inspect the extrusion of the top layer of some of your prints: If the top layer looks fine, your bed is leveled too close; however, if the top layer is over extruded as well, then your feed rate is too high.