It seems to me that your typical Heavy Xeno form (i.e. having several levels of unnatural toughness) has far more durabilitiy against your typical heavy weapon than due heavy vehicals, due to the fact that most heavy weapons rely on a high penetration to negate armor. Which is all well and good but bio-titans rely on their toughness to negate damage rather than a high armor (I think the highest a tyranid has is 12, with a TB of 21 or so). It does not seem to make much sense to me that a living creature would be more resistant to a Las-Cannon than a main battle tank. Plus most vehicals have far fewer wounds than do a bio-titan. The felling (X) quality helps but its very rare on published weapons.
Thoughts and suggestions?