There are lots of places where you may be 'losing' fabric - in addition to the seam width
Are you cutting your pieces 'skimpy'?
Are you using thick thread in your seams?
How are you pressing the blocks?
Do a test to see where the problem might be:
1) Cut three strips of fabric 2 x 5 inches long
2) Sew them together with 'your' 1/4 inch seam
3) Press
Measure - the unit should measure 5 x 5 inches - and the center strip should measure 1 inch wide
This will only take you a few minutes to do. If the finished piece is too small, make your seams a tiny bit narrower. Or place your ruler so that you 'include' a bit more fabric in your cut.
Then try it again, to see if it's better the next time.
Are you using templates? Just 'strip cutting'? Using a June Tailor shape cutter? A die cutter?
'Scant' isn't really that big of a deal. If one drew a fine pencil line 'exactly' on the 1/4 inch line - one would stitch on the outside edge (closer to the edge) of the pencil line.
There are a lot of places where things can go awry. I think one reason that some patterns say 'scant 1/4 inch' seams is that the pattern is comparatively finicky.
I think some sewers think of seam width like some people do with medicine - if some is good, more will be better.