Mathematically, not too big means that one number is less than or equal to another number, that is, ≤0, or symbolically expressed as X ≤ Y, which means that X can be equal to or less than Y. For example, we say that a number does not exceed 10, which can be 10 or any number less than 10.
Mathematically not higher than, it also means that one number is less than or equal to another number, that is, ≤ 1, or symbolically expressed as X ≤ Y. This means that X can be equal to or less than Y. So from a mathematical point of view, there is no substantive difference between not exceeding and not exceeding. They all mean that one number is less than or equal to another.
However, semantically, they may have some subtle differences. For example, when we say that the price of an item does not exceed 100 yuan, it may imply that the price of the item may be close to 100 yuan, but it will not exceed 100 yuan. When we say that the price of an item is not higher than 100 yuan, it may imply that the price of the item may be lower than 100 yuan, but it will not exceed 100 yuan. These subtle differences may depend on the specific context and the speaker's personal interpretation.
To sum up, mathematically speaking, there is no difference between not exceeding and not exceeding. But semantically, they may have some subtle differences, depending on the specific context and the speaker's explanation.