Text styles can be specified for each layout on your field while in layout mode and you can also have text that has a style assigned to the actual text in your text field. Text that is copied and pasted on Mac systems or which was selected and assigned a style while editing might show such results.
If a given field has exactly the same style for every record, this is likely a style specified for the layout's field. Enter layout mode, select the field and use the formatting bar or the Inspector's appearance tab to select the desired font size and any other styles.
If only some records show this style or the above layout based change does not change the style, the text within the field may have a style assigned to it. In which case:
Show All Records.
Click in the field and select Replace Field Contents from the Records menu
Use the calculation option with this expression to strip out all text formatting:
TextFormatRemove ( Yourtable::YOurFieldName )
And to prevent such text formatting on text in the field, you can use this auto-enter expression on the field:
TextFormatRemove ( Self )
and clear the "do not replace existing value..." option.