As Mario, juggling is key to winning most battles and is why he does so well against heavies. Snake, although having a load of projectiles, is still a relatively big and heavy character. This allows Mario's up-air strings to flow a lot better and kill with up-b or forward-air more often. Keeping Snake off the ground is great as well, his off-stage and midair presence is below average, so up-air strings are usually what you need to get Snake to higher percents and even kill. This is one - if not the - best tool Mario has and is a great part of his meta-game.
Mario