Light vs Medium is mostly just a build choice; Heavy armor is somewhat more powerful.
Medium and Light armor both max out at the same point, AC 17. In Medium armor, that's achieved by the relatively simple means of having a +2 Dex bonus and wearing Half Plate armor (expensive but not ridiculous); in Light that's achieved by wearing studded leather and getting the maximum score of 20 in Dexterity for a +5 bonus, which may require many levels depending on your race and starting ability score. For the purpose of this discussion, I'm ignoring magic armor, which applies to all armor types equally, and effects that would allow a character to break the ability score maximum, which are typically what you might call "end-game content".
Medium armor is potentially "more powerful" than Light armor because it requires a smaller investment of resources to get to that maximum AC of 17. This makes "full AC" more readily available to a wider array of characters -- everyone has gold, and a 14 in Dexterity is available to virtually any character if they really want it.
For races that lack a racial bonus to Dexterity, such as the Hobgoblin, Medium proficiency is a slightly more valuable armor proficiency than Light. Without a racial bonus to Dex, reaching that maximum score of 20 will be more costly and time consuming, and if your Dex score is below 14, Medium armor allows for a better AC than Light armor does, both overall and dollar-for-dollar.
That said, Medium proficiency is already widely available. There are many classes that start with it, and if you're allowed to use multiclassing, a single level dip into the vast majority of classes will grant Medium proficiency if you really want it. In general, adding Medium proficiency instead of Light at the race level only really benefits a handful of classes, such as wizards, sorcerers, most warlocks, and some bards.
By contrast, Heavy proficiency is definitely more powerful than the others. While acquiring a suit of full plate is fairly expensive, the AC 18 that it grants is beyond what's available from any Medium or Light armor, and it's available to virtually any build. There is a minor price in speed if you aren't pretty strong or a dwarf, but assuming you're willing to deal with that, there's not much of a downside to wearing Heavy armor.
It's not accidental that Heavy proficiency is relatively hard to get. It's available to only a few classes to begin with, none of which offer it as a multiclassing proficiency. Gaining Heavy proficiency after level 1 requires a valuable feat slot, or the use of a non-obvious backdoor by multiclassing into one of a very small number of specific class options that will grant Heavy proficiency through a class feature (most notably, multiclassing into Cleric with an appropriate domain choice). I feel that the relative rarity of Heavy proficiency and difficulty of acquiring it is the developers telling us that Heavy proficiency is a meaningfully powerful class feature, and that in itself is a good enough reason not to simply hand it out to an entire species.