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Link between passive tag size or its antenna and reading range

Thoose days the hitachi's u chip is the smallest known microchip on the market . There are controvercials for different reading ranges . The general opinion is that the tag's  read range is within few centimeters . 
Why so , and why you cannot increse its range . Is there some fundamental restrcition due to size of antenna and wavelength that does not allow bigger range to happen ? 
I will be glad if you put some math in the explainations 
Thank you