The phrase "when you give them an inch, they will take a mile" illustrates a type of flawed reasoning known as the slippery slope fallacy. This fallacy occurs when it is assumed that a relatively small first step (like giving someone an inch) will inevitably lead to a chain of related events (taking a mile) without sufficient evidence to support that progression. It suggests that allowing a minor concession will lead to much larger and often undesirable consequences.
This kind of reasoning can also reflect overgeneralization or hyperbole, implying that one small act will necessarily lead to excessive or unreasonable demands later on, which may not always be the case.