According to the tutorial the Heavy worm is "strong" and the Scientist and Scout worms are "weak". However so far I haven't found out exactly what it means by that...they start with the same health and appear to take about the same damage from attacks. What does the "toughness" of the worms do for them? I also know there are some other effects like the Scout's speed and thin-ness and the Scientist's stronger turrets.
What are all the differences between the 4 worm types?