That sounds more like GRRM is writing Syrio as saying their swordfighting is inelegant, not that the weapons themselves are heavy. From what I understand, there was often a lot of hacking and hammering to get through the armor, even if the weapons were light. From here:

https://en.wikipedia.org/wiki/Half-sword

> Half-sword is used for leverage advantage when wrestling with the sword, as well as for delivering a more accurate and powerful thrust. Both of these are critical when fighting in plate armour because a slice or a cleaving blow from a sword is virtually useless against iron or steel plate.