This is not always the case....ad has been argued about often, in fact there is a rule/law in athletics that adult transgender females, must be on MTF-HRT for a minimum of 3 years, sometimes as long as 10 years before they are labelled medically as physiologically compatible as cisgender females.
Those that matured, within their 'natural' biology aka a female identified, male matured into adulthood or even as a senior will most likely maintain to some degree heavy parts of the 'natural' biology & at least in theory HRT will not work on them, they same that it does for those of us girls that are younger when on HRT.
Puberty blockers, will keep the child' prepubescent until they are legally and medically permitted to begin MTF-HRT.
That means that unlike peers their own age, a 15 year old MTF on puberty blockers, will have the prepubescant physilogy of a peer at the age in which they blocked puberty in almost all cases thats the strength-aka physiology of a child instead of a teenager.
There is also to take in consideration that MTF-HRT does change the mass majority of the male physiology to that mirrored of a female physiology.
Muscular atrophy, especially in biceps & penile, greatly decrease to almost nonexistance.
Building muscle will be much more difficult for the MTF then the cisgender female, due to the HRT counteracting those physiologically inclined male characteristics.
MTF-HRT works in two important parts. Oestrogene & or progesterone...and an anti androgene...the anti androgene blocks the production of testosterone, more often then not, less measure then a cisfemale in order to transition physically and change without testosterone battling the estrogene, which if left without an antiandrogene would almost completely overmeasure, thus prohibiting any transitional effect more then moodswings.
Alot of thus, will undergo an orchiectomy so they will not need to use antiandrogenes, as then testosterone production is cut even bioligcally to almost nothing.
Though many especially those that know this...will keep their scrotum for the SRS which the skin is used in/for Labialplasty (creation of the labials) in the standard form of SRS.
An orchiectomy even then increases and quickens physical transition....so its technically a scale issue in decision making.
There are also other instances in which change this capability'.
Bioligcal, medical, postoperative etc abnormalities; can and or will void those opinions.
Many if not all transgender females intentionally do not work on gaining muscle...after puberty this in turn can make it almost as difficult gaining muscles like that of their own bioligcally inclined cisgender males.
Myself....I spent mass majority of my teen years in and out of hospital.
Life saving operations, allergies, postoperative complications etc.
This in turn left me frail, I have almost NO immune system, I am almost always ill.
I am physically weaker then my bestfriends' 10 year old nonathletic daughter.
The real reason that transgender females, are argued about in sports is because cisgender females do nt have to prove their womanhood, thus transgender females work harder around the clock to prove to society especially peers and family that they are without a doubt female.
For some like myself, I prefer the nonathletic girly girl way of life, I have no problems asking for help from men when it is beyond my capabilities...like mass majority of cisgender females.
Others, especially if athletic, will do all they can to prove their femininity to be the best woman for the job...thus work harder at it....woman/girl power, feminism at its best.
So as you can see there are many variables that discredit that hypothosis given as fact, regardless of medicaly proven research.