The nation's tata is simply a slang term for a breast that, although it may be a bit on the small, or perhaps overly large side, is still leaning towards average. You might say that a breast that is the nation's tata is still easily squeezable, played with, etc.
Although my chest isn't that big, I'm still nation's tata.

Although I surprisingly wish I was a bit smaller in the chest area, I'm still nation's tata, so it's not that much of a problem.
by Vacuum Sweeper #5 May 25, 2005
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