Does Bottle Size Effect A Wine’s Flavor?

No, there is no taste or quality difference in wine bottled in a magnum or any other large format from those bottled in a traditional 750ml bottle.

However, wine in bigger bottles do tend to age more slowly than in smaller bottles. This is supposedly since the amount of oxygen trapped inside the bottle isn't as great.

Occasionally, if you see the results of a blind retrospective tasting of older wines, it is often mentioned if it was tasted from magnum bottles. This takes into account that it might not be showing its age as much as if it was tasted from a 750ml bottle.