I highly suggest converting to pellets. You say you feed seeds with fruits/veggies, but the plastic dish pictured has round, flat, rainbow pellets. Not sure what's going on here. The dyed/flavored pellets are not very healthy, I recommend Harrisons, roudybush, or almost any other unflavored and not dyed pellets. Cockatiels are grainavores so they should have seed in their diet, too, but not only seed.
That size of poop is definitely a normal sized morning poop, but you say it happens all throughout the day. Does she ever have smaller poop or is it only the big ones? How often does she poop?
Sometimes females will hold their poop if they are thinking of their cage as nest and getting ready to lay. Then when they do go, it is larger than normal. Has she been acting hormonal lately? As mentioned, morning poops are often sometimes large. How many times a day is she pooping? Does her vent look okay? Is she flighted? Also, you should try to convert her diet to include some pellets. Any fortified kind she will eat is better than no pellets.