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.
The only enjoyable line from the movie "Officer and a Gentleman", an otherwise insufferable Chick flick Can be lower case, or UPPER CASE, (self explanatory)