The code you have will make a new Rectangle at x,y -10 compared to the sampleRect. To compensate you increase the Width and Height with 20.
I assume you are trying to increase the rectangle around the center, in that case you need to move the rectangle half of the new increase.
Example:
var sizeIncrease = 10;
var newX = sampleRect.X - 0.5 * sizeIncrease;
var newY = sampleRect.Y - 0.5 * sizeIncrease;
var newWidth = sampleRect.Width + sizeIncrease;
var newHeight = sampleRect.Height + sizeIncrease;
These values should give you what you are looking for
Rectangle.Inflate will also change the size around the center.