Hi Jillius, I would say that if your yorkshire's are soggy underneath, it is because the oil or the oven wasn't hot enough when you put the batter in, and not because you had put too much batter in the tin.
For best results, they should be cooked in white fat rather than oil (this is because white fat gets to a hotter temperature than you can get with an oil). I am vegetarian, so I use either PURA or TREX (which are made from vegetable fat). Don't use Self-Raising Flour, they should be made with Plain Flour. Using two eggs makes them rise a lot! Also, a milk & water mix makes them light.
Individual Yorkshire Puddings
4 ounces of Plain Flour
2 beaten eggs
Quarter of a pint of milk & water mixed together(half & half)
Pre-heat the oven to Gas Mark 9
Put a little white fat into (about) 8 sections of your bun tin and put the tin in the middle of the oven to heat.
Make the batter.
Mix the flour with the eggs and milk mix, beat to combine, (I strain this through a sieve to ensure no lumps, into a glass measuring jug - which makes it easy then to pour). Pour equally into the sections of the tin, put back into the oven, and turn the heat down to gas mark 6. Leave until puffed up and golden brown. As you take them out of the oven, take the puddings out and place on a cooling rack - don't leave them in the tin, this will also make them go soggy.
Good luck.