Until 2.1 the big ships could not be challenged 1v1 (assuming competent flying on both sides) but they would lose more than the value of their base shield in a hard-fought duel against a medium ship, making that back up via SCB's.
The Feedback Cascade Rail, which blocks 90% of the operation of the SCB, has overturned things, meaning that at this time the Fer-de-Lance is better 1v1 (although its shield numbers are lower than the 'big three', its effective shielding is far higher because it can still use its SCB's - the big ship can't hit it with the rail, if chooses to move out of line or out of range).
Effectively the big ships have lost that upon which they depended - their SCB reserve.
This vid shows Cmdr Oddisee of SDC (a very experienced duellist) v Cmdr Majinvash (also very experienced and former Captain of The Code), FdL v Corvette. Note the relevance of the rail.
(Um... advisory ... 'raw' rap lyrics!)
https://youtu.be/M8xD8PzHXsg
However, considerable changes are currently taking place in Beta 2.2.03 so we will have to wait and see whether big ship supremacy is about to be restored.