Depends on what you mean by "the same shape". Does the image have to be the same size and shape? If you want approximately the same shape of the white blob, but are willing to have a larger image, then you can use imresize() like Stefan suggested.
If you want the same size image but just have the white blob larger, then you could use imdilate():
newMask = imdilate(mask, true(11));
However this will round out the shape (smooth it) the larger it gets. If you want a larger white blob but the same size image, you can do it in two ways.
One is to call imresize with the nearest option and then crop the image. You'll have to figure out the rows and columns to crop it at.
The second way is to use bwboundaries() to get the x and y coordinates of the perimeter, and use regionprops() to get the centroid. Then calculate the distance of each point from the centroid and extrapolate that outwards by some scale factor. Now you have a new set of expanded x,y coordinates and you can use poly2mask() to turn them into a binary image. This should be perhaps the most accurate way to retain the exact shape.