Originally in the 18th century a grenadier was exactly as the name implies ... a squaddie whose job was to throw grenades (not dissimilar to the bombs supplied to Wyle E Coyote by the ACME bomb company).
To throw the bombs out to a safe distance, the grenadier had to be the biggest and strongest. This led to one company (the grenadier company) in a battalion being full of 6 footers who were much rarer than they are today. The smallest and quickest in turn ended up in the skirmishing company where being nimble was an advantage.
As time moved on into the 19th century and the Napoleonic wars, the grenadier company evolved into the nailsest 120 blokes in the battalion and tended to anchor one end of the battalion in line. They would also be used as the first unit into an assault.