Because of time considerations ?
I made a 1 and 1/3 recipe last night which seemed to work well even though I accidentally made a couple mistakes (like I forgot to put in the last 2/3 cup of sugar & mixed half a pkg of 1 brand pectin with an eyeballed "half" of different brand that I had just bought...etc.)
Larger scaling, like double, I have never tried, but I thought it was discouraged because of the timing with hand canning: You will either have to leave the extra jam in the pot twice as long with the fire off, potentially getting cool; or keep it cooking the whole time, and it may over-thicken.
The ingredient ratios themselves really can't be all that exacting, or recipes would routinely fail due to years of slightly sweeter, juicier, or tart-er fruit, or different varieties of trees, etc. Not to mention forgetting abt. 6% of the sugar, which didn't seem to matter at all.