This could be "Die Swell" where something that's cold extruded through a nozzle swells in size after it comes out the nozzle. While this is definitely an extreme example it is possible to get 2x expansion.

Check your temps and the temps of the new material.

Edit: are you sure you just haven't bought some 3mm filament as the picture doesn't look extruded, it looks like those are filament off the roll.