I think I don't understand why the future leader of the armed forces needs to go through physical service when she is a small girl who is likely going to be a very petite woman. (She's not so small that she'd be disqualified from serving in most places, but can't they train her in intelligence? Or... anything else where her physicality isn't going to matter? There's more than combat, even in wars.)
That being said, there are some rather small female soldiers with large weapons in a number of countries. I'm sure Spain is one.
I allowed myself to divide your post into two segments.
I actually think it is important for someone who is going to be the head of the armed forced to have a personal connection and personal experience of the military life and the people he/she is head of.
It is IMO a question of showing respect for those who serve in the armed forces. And getting respect in return.
It's also symbolic. A commander of the armed forced who hardly has a clue what military life is and how the whole thing functions? - Come on.
Leonor and other future heirs don't really need to have a military career - that is recruit, cadet, officer, staff officer, general. But they need to know about the life in the military.
Leonor will not have time to have a full professional career in the military. It's unrealistic to expect that. She will have to study all sorts of things at universities and travel and meet people, not least the Spaniards and not least, gain working experience as a royal, both with her parents and alone. - And at some point find a suitable husband to ensure the future of the dynasty.
So no matter what, she will jump in and out of the military until it becomes her turn to take over.
So that will be classes in logistics, intelligence, staff work, basic pilot training, basic medic courses, parachute jumping, service aboard a patrol frigate and that sort of things, that will teach her about military life in all the branches but is not a career.
Then we return to the physical aspects. Oh yes, there are many small women (and men) serving in the military. It depends on their functions and how they are build physically.
A girl who is 1.57 can be a powerhouse of strength, while a girl who is 1.80 can collapse the moment she puts on her backpack. It's physique and will-power. Many recruits have the will-power but they don't have the physique. And vice versa.
And looking at Leonor now, will she grow a soldier's physique in the 3-4 years she has left until she has become a full grown adult?
And if she is to be an officer, she must lead her troops from the front - all the time. She can't lead from the back, half-dead from exhaustion. She will lose the respect of those she commands.
I want her to succeed in the military. I want her experiences there to be something that gives her self-confidence and develops her as a person. Not something she dreads and hates and where she suffer defeats, simply because she is physically unable to keep up.
When looking at her she is a runner, not a weight lifter.
She's agile but not strong.
That's how she is constructed from nature.
So let her service be something that suits her physically. But being a career-officer in the army or airforce (where you as an airman also need physical strength)? - IMO forget it.
She can indeed opt for a more specialized career. Logistics, intelligence, air defense, drones - but that's a more technical position, where she needs constant practice and schooling to be of use. And her career will end around the rank of major or lieutenant-colonel. I.e. short of ending up at a general-staff, because she is not getting direct command experience over combined units.
There is one last factor, the Spanish members here can answer:
The Spanish press can be pretty pity-less! How would the Spanish press react if Leonor is send home from NCO school or the officers academy because she couldn't cope with the physical demands?
I guess I'm just confused- as someone who did serve, I had Leonor's physique at her age and made it through basic training without any problems (other than the normal foot blisters and heat rash). I don't think she has any obvious defect that will prevent her from serving.
Most people do some basic workouts before entering boot camp, and she has many years still to go. I wasn't training for boot camp at 15 by any means. When the time comes, I'm sure Leonor will be prepared.
Bootcamp is tough but it's definitely doable. There are special forces with their own strenuous qualifications, but many, many physically fit men can't pass those quals. I don't hold that against anyone.
In my opinion, the hardest part is mental, not physical. And Leonor will be following in the footsteps of at least two Princesses with military training (who also appear to be slender like her). Princess Elisabeth, Duchess of Brabant, and Princess Salma of Jordan
I'm sure Leonor, barring injuries, can complete boot camp. That's after all designed so that most reasonably fit average persons can complete it
But there is a difference between being a recruit and then a private and being an officer.
I don't know Salma of Jordan. But I have seen Elisabeth of Belgium and to me she looks athletic. - And the path she has chosen won't be a picnic for her, despite obviously enjoying that lifestyle.
Let me be clear: I will happily post a photo of myself lying prostate in the mud in growling admiration of Leonor, should she make it through the physical ordeals of boot camp, officers academy and serving first lieutenant.
I just fear the odds are against her, physically.