What Linux OS is the lightest?

I have a Dell Mini 1200 notebook. It is very well made, but tends to overheat. Has no fan, and an Atom 1.6 Mhz single core CPU. I am replacing the hard drive with a SSD which should improve things. It came with Windows 7. I have been trying different Linux distro's, and find that FatDog 64 is lighter on resources that MX-19.1, or Mint fxce. I just need something to run a browser. Can not convert to Chrome OS as it will not boot on the Atom processor. Some of the Puppy's are light, but lack up to date software.