Usually each store has its own sizing chart.

No. All jeans and dresses are different. Some dresses run small, some run big. Same with jeans. it depends on your shape! My dress size is actually slightly smaller than my jeans size!

People who are pear shaped notice that jeans tend to be very tight on their really big thighs although they have very small chests,small shoulders and normal waist so dresses need to be quite small.

So ya, it depends on whether your top or bottom half is bigger. Dresses fit the top half more closely, so if you are smaller on top, you would have a smaller dress size and opposite.