To expand my comment, semantics is simply the meaning of words:
the branch of linguistics and logic concerned with meaning. The two main areas are logical semantics, concerned with matters such as sense and reference and presupposition and implication, and lexical semantics, concerned with the analysis of word meanings and relations between them. Source
See this pdf.
Not everything in English is literal - consider humour, irony, sarcasm, personification, similes, and metaphors.
A simile (like your example) is:
a figure of speech involving the comparison of one thing with another thing of a different kind, used to make a description more emphatic or vivid (e.g. as brave as a lion). Source
In your example 'You are as fat as a pig' means 'you are very fat', so fat that you resemble a pig. See https://idioms.thefreedictionary.com/fat+as+a+pig.