My experience after orchiectomy 12 years ago was a slight drop in strength and maybe a slight drop in muscle mass 1-2 years later. My whole life I have been very active (cycling, mountain climbing, hiking, weight training, etc.) so when I started transition I was lean and had solid foundational muscle though I was never 'bulky' and tended toward the thinner side. To this day I remain active and go to the gym 1-2 times per week in addition to the usual athletic pursuits.
Despite orchi, estrogen injections weekly, with periodic finasteride/dutasteride, I have not appreciably seen a decrease in my overall fitness or competitiveness in sports. Last weekend I climbed a 9,000 ft summit and in two weeks I'll be climbing a 12,000 foot peak.