The geometry of a P-pickup coil tends to pick up more fundamental than overtones (go ask Rob Turner at EMG why - he used to have a line in the "faqs" section that talked about resistance v impedance and why coil geometry makes different pickups sound different.). The thicker the string, the inherently less flexible it is unless the construction is internally compensated. This translates into the string not vibrating with the same balance of overtones, hence the perception of "thumpy" or "tubby" tone. Some companies simply use a thinner E string than the math otherwise would indicate; this includes Fender from the beginning, using a 95 or 100 E string on a 45's set or even sometimes a 50's set. Other companies, like DR, have multiple windings to keep the core as thin and therefore as flexible as possible. Jon Moody of GHS has made great strides in the next generation of "balanced" strings that keep the consistency of tone with consistency of tension and feel throughout the set. Even with the D'A EXL170BT 45-107 set that I really enjoy, I keep extra 107's, since for me the E string does still tend to die before the rest of the set. But that may be my particular body chemistry where my perspiration tends to eat nickel. The set that lasted me the longest was when Fender still made the 9050CL flatwound set in 45-105. I gigged that one set for over two years changed the E string for an Ernie Ball 105 stainless flat (NOT the Cobalts), and gigged the set another year before moving on. Now all you can get is the 9050L set, which the E string lasts longer, but I just don't like the feel so I have another EB105SF in reserve.