Synthetic insulation is lighter than fleece for the same insulation value, traditionally a 'mat' of insulation there's now even more efficient 'loose fill' insulation that mimics down.
Synthetic insulated garments outperform down in wet conditions, a 'mat' format is tried and tested but 'loose fill' insulation is getting close to the performance of down. There's also a move to 'breathable insulation' such as Polartec Alpha, not as warm as other synthetics but better for faster moving acitivties.
