(assuming you're using Responsive Dashboards and glide.cms.enable.responsive_grid_layout=true)
When the widget sizes don't match it is usually because A) a homepage is converted to a dashboard, or B) a tab and its content are converted from a Geneva version, or Helsinki version without Responsive Canvas, or C) the sys_grid_canvas_pane and/or sys_grid_canvas records are missing in the unload.
The sys_grid_canvas_pane records are holding widget position and size information, so when that's missing or incomplete you get size and/or position errors.
However, when the sys_grid_canvas record is missing it tricks the system into thinking you're opening that tab for the first time since Geneva/Helsinki, and it makes it convert the old layout system (when there is one) to the new responsive layout.
Because the legacy layout (based on UI Macros) used in both home pages and Geneva/Helsinki dashboards relies on HTML markup, which may not always correctly convey layout structure and widget sizes (DIV elements convey no structure at all, but TABLE elements do), we are forced to round off the results to the closest match in the new responsive layout during conversion. For instance, when a widget's width corresponds to 2.5 "responsive columns" it's going to be rounded off to 2 or 3 columns, and so will either be too wide or too narrow compared to the original size.
The height of widgets is yet another problem. If their height was not explicitly set as a fixed number of pixels, we have to give them a default height since the widgets HTML+CSS+JS can't be rendered server-side to find out what their size would be when displayed on a browser.
I know this won't take away the frustration, but I hope it at least explains what you're seeing.
(more about responsive layouts here)
