I'm going to assume that you mean 17-18th century European grenadiers. A typical grenade at that time was a little bit bigger than a baseball and was a hollow iron ball filled with gun powder along with metal shards or stone to provide shrapnel when it exploded. A fuse stuffed the hole at the top where the gunpowder and shrapnel was put through.

The reason why a grenadier had to be strong was twofold. The first was that they'd have to throw the grenades a fair distance. A muscular grenadier could throw his grenade far enough to the point where an enemy picking it up and throwing it back to cause harm would be minimal. Going in closer to the enemy also meant that their fire would be more accurate, necessitating the farthest possible distance possible.

The second reason is a side effect of the first reason. They were considered elites and thus intimidating. Because grenadier regiments had to be comprised of generally strong and tall men, this created an intimidating effect on the enemy. Grenadiers had to be well disciplined in addition to their strength because they needed to remain calm when throwing their grenades under heavy fire and retain that discipline if their buddies grenade went off right next to them due tot he fact that he was shot when throwing it. This discipline along with their natural intimidating physique meant that grenadiers would be considered elite units and thus necessitated certain characteristics to keep that status.

As you noted a big and strong guy can still get shot and that is the primary reason why they fell in use after the early 18th century. Infantry line tactics improved to the point where a huge volumes of fire spelled death for grenadiers who got in close. The need for elite shock troops remained however since enemy lines still needed to be broken after they showed signs of wavering. Grenadiers combined the discipline needed to charge into a large body of troops and the power needed to be effective in a skirmish. Thus grenadiers still needed to be fielded to some extent.