I did a quick comparison between mozzies and express and find that the express are slightly shorter. Mozzie 5.50mm, express 5.38mm.
I haven't compared weights but with some weighed Exacts the ones that are lighter than 0.550gms are shorter (slightly) and those heavier (0.56gm) are slightly longer. Which kind of makes sense in that the wire stock(?) is cut reasonably accurately but there will be a variation. Head size is hard to measure since it needs a jig of some sort to keep the pellet at 90deg to the vernier caliper (I used to have a good quality micrometer but haven't been able to find it in years, sadly) but even so the caliper only measures to 0.01mm so everything is +/- 0.01 at least). I didn't see a huge difference between the mozzie (4.46 to 4.48mm) and express (4.46 to 4.48mm) and the exacts all seemed similar (and it gets boring very quickly)
So variation in length equals weight which means small changes in fps but I would imagine that variations in length will produce effects which differ from barrel to barrel, differing BC, differing who knows what :-( However, for those that weigh it may help to improve consistency if pellet length matters while measuring the head dia. is harder but again may show difference between batches/dies.