Indeed.
Just do these to various degrees.
- Same or at least nearly same XP buff for everyone. Make it 20% for 1man, 30% for 2-4 man, that is a good start.
- Bounty adjustment for 1/2/3 players. More bounty rewards for each of these group sizes.
- Reduce monster HP for 1/2/3 players.
- Buff follower with new/better items. Make them immortal as default.
Patch 2.7 re-balancing mega wish-list - some ideas for follower items/buffs in this thread