Yes, Jeffries did get better as he progressed from the early days of Choynski were he was basically hustling himself through the blood with toughness and strength. Jeffries was always big and powerful, but we must remember that although he was one of the first true, big-bad bears, if you will, he was a very placid man and could be in the ring also. He was particularly scourned for not getting rid of Sharkey in the rematch were he was not thought eager to close the deal. Lacking that 'true malice' some thought. Jeffries was a strong but patient fighter with the left out there and feeling his way through. He likely steam-rolled his first few men and then realised that he needed to put the breaks on a bit when he fought the higher-calibre men and so injected feints and tactical observations before pouncing. Also, remember the era we are talking about here. Small gloves made for hellacious body shots. Fitz may have been one the 'smaller big boys' but we are all well acquainted with his reputation and a shot to the stomach off him in '5 ouncers' would not have been what the doctor ordered. Through Jeffries speed, improved crouch and short punches he made himself into a kind of tactical work-horse who could blast you out or grind you down. There are a few parallels with the way in which Marciano adapted his forth-coming ways as new hurdles arose.
Jeffries/Edgren 'My Life and Battles', 1910 (cited Pollack 'In the Ring with James J Jeffries' p. 217) "Two days before" (Jeffries-Fitzsimmons I) "I weighed just 206 pounds stripped, and let everybody around the camp see me on the scales. The day before the fight I went with a number of reporters to the baggage room at the railroad station. There on the baggage scales, in jumpers and a light sweater, I weighed an even 215 pounds. I never attempted to make such a low weight again, as I know I'm stronger and have more endurance when I carry forty punds more flesh on my bones." (As always, I highly recommend all of Pollack's 'In the ring with ...' series).
If I remember correctly, in Pollack's 'In the Ring with ... Jeffries', Ruhlin is cited, after both his fight with Fitzsimmons and his rematch with Jeffries, to the effect that early in each bout he took a body shot which nearly made him black out. If you read the books account of Ruhlin-Fitzsimmons, it's clear Ruhlin took an absolutely horrendous beating.
Nice thread I just wish I could see Jeffries fight someone in his prime, when he could dance around the ring