Because clearly, he woudn't have commented, "She's in such good physical shape" just by looking at her, would he? Surely, our president knows that being "in shape" is not attached to a certain look - or shape or size. It's not about structure or form at all. It's about the ability to perform various physical tasks at certain levels and about the body's physiological response to performing these tasks. We might want to assess data points such as her maximal oxygen consumption during exercise and the rate at which her heart rate returns to baseline post-activity.
One's weight or size is easily independent of one's fitness level. There are plenty of thin people who have zero physical training and plenty of fat people who run marathons and hoist heavy things above their heads. What we typically think of as physical indicators of fitness (e.g., a toned body, obvious musculature) are correlational at best. Many individuals are "blessed" with thin or muscular genes without ever needing to hit the gym.
Now, if Madame Macron is, in fact, no stranger to Velib, can lift even half of her body weight repeatedly, or knows a thing or two about downward dog, we might say she's in good shape. But, without this information, her being "in such great shape" is pure conjecture.
What Trump really meant to say was, "You're so thin" before he called her "beautiful." Wasn't it? That's all that he observed. And what he could have done, as a statement to women worldwide, was learn a little bit about her beforehand so he could comment on something more meaningful than her body or simply mention how pleased he was to meet her.
You can find Does Every Woman Have an Eating Disorder? Challenging Our Nation's Fixation with Food and Weight on Amazon (as a paperback and Kindle) and at BarnesandNoble.com.