Why are these power cables different?

I had a 450w power supply by CoolerMaster and just puchased a CoolerMaster 600w.  They are both ATX form factor.  The product numbers are:

450w: RS-450-ACLX
600w: RP-600-PCAR

I've included a photo of the two.  The smaller plugin is from the 600w P/S and doesn't fit my motherboard.  They are both ATX.  Why does the 600w have a smaller plugin?

Photo: https://filedb.experts-exchange.com/incoming/ee-stuff/5781-photo.zip 
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PUNKYConnect With a Mentor Commented:
One is 24 pins and other (old) is 20 pins. Both will fit in power connector on motherboard, but if you use 20 pins, you will leave 4 posts opening.
brettrAuthor Commented:
I see it does fit.  Is there any disadvantage to using the 20 pin?
-DJL-Connect With a Mentor Commented:
There should be another small 4pin connector that slides onto the side of the 20pin connector to make it 24pin.

I wouldn't power up the motherboard with only the 20pin connector, you may damage it.  

If you can't make the 24pin connector, have a look at the motherboard manual, some motherboards with 24 pin ATX connectors will allow you to run with the 20pin supply, but only if you connector other power cables to the motherboard.
brettrAuthor Commented:
Yes, thanks.
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