Dell E310 - toner does not work?

Hello to all,

I have printer Dell E310. I bought new original toner Dell E310/E514/E515 Series P7RMX, but it is not possible to put this toner into the Dell printer. It seems that this toner is too small to fit there. Where could be problem? The previous toner was Dell E310/E514/E515 Series KJ0Y3, much bigger size. What does the P7RMX and KJ0Y3 mean?

Thank you for your help and answers.

V. Vavra