> since this is svg output the top left of the document should report as (0,0)
Since the GUI uses a differently oriented coordinate system (first quadrant of the normal Cartesian coordinate system), the "fixed point" when changing the page size is not the SVG origin (originally at the upper left corner of the page), but the GUI origin at the lower left corner of the page.
(Yes, requests to use the same coordinate system for the GUI as defined in the SVG specification have been filed multiple times, and the feature is on Inkscape's roadmap).
> When I resize a document I keep no memory of the previous
> document size and so the svg output becomes confusing.
Not within Inkscape's GUI though (unless you are in advanced editing mode and add hand-written code via XML Editor, or edit the SVG file later-on externally).
Tip: To get "rid" of the transform attributes on top-level layers, create new layer(s) after the page resize, and move the content of existing layers to new layers (which won't have a transform attribute until the next time the page height or orientation, or the position of the lower left corner (resize to selection) is changed).
Note: after moving a selection of objects to a new layer without transform attribute (with 'Shift+PageUp/PageDown'), the layer transform will be transferred to the individual objects of the moved selection. Depending on their type,
- this transform will stick (inkscape shapes, clones, offsets, …)
- can be flattened by ungrouping and regrouping (groups)
- can be optimized (flattened) by forcing a rewrite of the path data e.g. by nudging the selection with the arrow keys (regular paths only)
You can also create custom templates with your preferred page formats, and remove the 'transform' attributes of the default layers before saving them into '~/.config/inkscape/templates'.