The “Columns” option for FvwmButtons holds the number of button widths along the width of a button bar. You declared:
*FvwmButtons: Columns 1280
which means you wish to have 1280 buttons along the width of the screen. Since I take it that your screen is 1280 pixels wide (because you delcared the geometry of the FvwmButtons module to be 1280x24), you have made with this “Columns” option 1280 buttons, each 1 pixel wide. The geometry for single buttons within the module are given as button widths and heights. So your swallowed xclock here, with the geometry 205x1, has a width of 205 button-widths (that is, pixels as you have it set up). You might rather have something like:
*FvwmButtons: Columns 53
if you want square buttons in your bar. This is 1280/24 (24 from your overall geometry specification) rounded down. It would be larger if you wanted rectangular buttons.