Did you compact and do a comparison? The info would help to know if it's just a matter of overnight data entry. Is the db run at night by any updating process? There are several reasons why bloat happens, but which might apply depends on factors not revealed here. Attachment fields? Image fields? Design changes are obviously out of consideration, but over-writing tables is not so obvious to us. If nothing is happening overnight, suspect incorrect reporting of the file attributes by Windows. Rather than rely on Windows Explorer, open the db and get the file attributes from within. Consider a compact operation on close if the db reaches a certain size only, which you determine. Something that won't occur every time. I think your example is just under 4Mb which is not bad - unless maybe that represents a huge percentage of its original size and/or is happening every night.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.