Bruins have been in the market for LHD with size for the past few seasons, so I think it's definitely a check list item for them that they finally have. And while I'm not comparing the two results wise, I think the Bruins see the value in Zadorov in how he has similar attributes as Chara and miss that component in their defensive structure - a huge guy that can physically dominant, intimidate, and hit some booming shots in for goals. How that's quantified on paper, I'm not sure.
With Swayman, there may be some hubris involved in thinking that with elite level coaching and defense first teams, they've been able to turn Rask, Thomas, and Ullmark into Vezina winners ( most Vezina wins in the league since 09 I think ) so they may feel that Swayman is benefiting from the system and is set to do so again with a long term deal.