Does anyone have the problem where when you're printing something very big (as big as the print bed), the print on the sides start to curl up? Does it have to do with leveling or do I need to change the tape? It seems to only happen to large prints.

I have the same problem too! What should I do?

The curling has to do with the plastic cooling at different rate. All plastic prints have this problem, and this is the reason why ABS printers require a heated bed since it has more shrinkage compare to PLA.

Try designing circles at the edges on the first layer of your print like the picture below. This will help reduce shrinking

I would say the plastic actually cools at roughly the same speed, but when it cools, it shrinks causing a lot of tension on the layers being printed and the ones below it.

I've attached a photo of what is happening. The first layer gets printed and it shrinks, but it isn't until the subsequent layers that it starts to pull upwards from the bed. As the 2nd layer gets printed, the tension inwards caused by the shrinkage pulls and puts tension on the layers below and eventually the corners lift. It also mostly happens on corners due to the fact that they are a single point, and has less lateral support from its neighboring materials to help keep it flat.

If you want to avoid your prints from curling, the most important point is making the distance between the nozzle and print bed 'just right', so that the plastic sticks to the bed well. The closer the nozzle is to the bed, the better it will stay down.

You can also enable the 'brim' setting which will create a ring around the object which also helps the print to stay down. The only downside is that you have to cut off the excess filament after it's finished printing. The top picture is with brim, bottom is without brim.