Fatty liquid vs non fatty liquid in dough I have been making bread at home over the past month or two using a simple 4 ingredient recipe with 60 - 75% hydration and have become familiar with the soft, light consistency of the dough when the gluten is fully developed.

The other day I tried an enriched dough for the first time, it included additional eggs, sugar and oil. I noticed it handled like a completely different animal.  It didn't feel as silky as the plain dough after kneading and felt a bit more dense and was not as fluffy when baked.

Since then,  I have been experimenting with different liquids in doughs with different fat levels. milk, buttermilk, oil, water. These have all turned out slightly differently from my usual bread and are richer but not as light as I've achieved in the past.  I understand that the water hydrates the gluten which makes the dough light and stretchy and silky, but what happens when a more fatty liquid is included in the mix, perhaps a mix of oil and water? does the fat inhibit the absorption of liquid by the gluten? The recipe usually calls for adding less water than I would normally because oil is included.  Does this reduce how much hydration I achieve? 

Is this the reason why these doughs tend to handle differently and not be as light, or is it possible to achieve the same consistency even when making an enriched dough?
The reason that your enriched bread handled completely different than your initial bread is because eggs, sugar, and oil all inhibit the formation of gluten. Sugar attracts water, so it competes with the proteins gliadin and glutenin in flour for binding to water (glutenin+water+gliadin= gluten) added to the mixture. 

Egg Yolks contain a high percentage of fat and liquid oil by definition is 100% fat. Fat uses a different mechanism than sugar to inhibit gluten formation. Fats coat the individual gliadin and glutenin proteins. Because fats are hydrophobic (not attracted to water) they effectively shield the gliadin and glutenin from water, which inhibits gluten formation.

With this collective reduction in gluten, the bread becomes much denser because the carbon dioxide produced by the yeast doesn't have enough gluten to use for expansion of the bread. If all other ingredients and factors were the same between your initial and enriched bread recipes, but you increased the amounts of sugar and fat the result is a denser bread.