So i recently fermented a pale ale but only was able to transfer just under 4 gallons of my planned 5.5 gallons into my fermenter due to transfer issues. My final gravity is shooting past my 1.018 estimate and is creeping towards 1.010 and is finally slowing down. So my question is with my lower amount of beer the reason my fermentation is going that direction?
No, batch size should not be what is taking the final gravity low.
I would question first why you expect an FG of 1.018. That estimate is likely inaccurate but depends most of all on the specifics of the recipe including grist, mash time, and yeast strain.
Also, you say you have brewed a pale ale. Assuming this is dry hopped, be aware that hops contain enzymes, which if not boiled will be present in the fermenting beer and can break down complex sugars into simpler sugars which the yeast will break down to more alcohol and carbonation. This effect is known as "hop creep", and is very likely part of the effect you are seeing here.
If you use the same amount of yeast in a smaller batch, you would expect a faster fermentation, and perhaps a lower final gravity. When making a strong beer (like a Belgian Triple, etc.) it is common practice to pitch a lot of yeast to ensure good fermentation that fully completes.
That said, a final gravity of 1.018 seems very high for a pale ale (on a sweet stout or very strong beer - sure). Whereas 1.010 sounds like a fine stopping point for a pale ale.