Heavy weapons are suppose to be unusable to small characters but in game i'm not seeing any difference.
Not quite - Smalls are meant to have disadvantage while using them, but they still can. The "Always Reckless" (advantage) greataxe-wielding halfling (disadvantage) barbarian, for example, can rest assured that she never needs to even think about advantage or disadvantage, because she will literally always have a straight roll, regardless of situation or circumstance.
However, in game it does appear that they've discarded this concept; smalls do not swing with disadvantage when using Heavy marked weapons - which makes you wonder why they coded in a 'heavy' category at all, since that is its only function.
