Nipple size/Breastshield Fitting ?

km3

I'm new to pumping and am using a Medela PISA.  According to the instruction booklet, my nipples are too small for the breastshield (funnel shaped piece).  I've pumped a few times with the size that I have and have gotten between 2-5oz per session.  Does it really matter that I'm not using the right size breastshield, or should I buy the correct size?  TIA.