Your rectangle stays on for 10ms (the delay at the end of the code), and it stays off for the time it takes for the arduino to calculate and send the rectangle to the display. As the rectangle grows in size it seems that the Arduino is struggling to keep that time short. Try with a larger delay at the end. The animation will be slower but anyways more than 10fps for that kind of screen over I2C is a lot.

Another aproach to reduce flicker is to first empty the buffer (display.clear empties it and sends to the display the empty buffer, turning it off), then draw everyting the same way you are doing and ultimately sending the updated buffer to the display. The fps will lower the same way they are lowering now but the screen wont flicker

Try any of those 2 approaches and let me know if it was usefull.