Are you talking about white wine in particular? I had a very interesting tasting of 05 Chardonnays over the weekend. Half the samples had not gone through ML while the other half had fully completed the secondary fermentation. Both sectors were dry (>2% RS) but the ML portions were as thick as Sauternes at this stage. My best guess so far is that the wines you are talking about have gone through ML, hence the increased viscosity. The lactic acid (same as in milk) is much heavier and can give you the impression you are describing.