That is pure FUD. Postgres is no more resource heavy than MySQL (the stock config will only use 32MB of ram!), and will perform better on many real world workloads.
I think it's not resource heaviness per se so much as high latency (and low user-friendliness) in a dev configuration. Postgres takes noticeable time to start up, both server and client, and the client feels less responsive; its commands are also more arcane (e.g. mysql's "show tables" is something like "\d"). Postgres is quite possibly better for a "production" configuration, but it's much slower to develop with, so developers get to thinking of it as "slow".
