THe reason for this is encounter assumptions are different for each system.What I have done plenty of is increasing the size of a number of encounters. There are a good deal more monsters in my conversions than there are in the original adventures.
In 3e: 1 Monster = 4 PCs*.
In 4e: 1 Monster = 1 PC.
*It's a hint more complicated than that, but adding monsters of equal CR to an encounter increases the encounter level. A single troll is a CR 5, so it's a normal challenge for a 5th level group, a level +1 for a 4th level group, etc. Add another troll and it jumps up to a 7th level encounter.