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the earlier the barlow is placed the glass balrow lens causes a diverging effect on the light cone [without it light rays converge quickly]-the earlier you place the greater the effect,it produces a larger image at the focal plane
think of a > being the normal light cone -then stretch it some for a normal barlow & stretch it some more for a before diag barlow
i have heard it can be up to a factor of 1x extra -but havnt botheed to calculate it myself eg 2x after diag & 3x before diag..
this of course relies on your barlow being lablled right -my GSO ED 1.25" 2.5x barlow is actually about 2.2x
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