Why does my body (measurements) change so much

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Whenever I work out I see firmer muscles in about 1-2 days but if I go a week without working out I look flabby and gain weight even if I eat below my limit. My measurements fluctuate daily, for example my calves can range from 12.5 inches-14. Can someone explain these fluctuations please. Also, I am lightly active daily as I walk to and from work and am on my feet and exercise 3 times a week.