Remember y is rows and thus will be the first index. So it returns an array 357*532*1072 which is 357 rows by 532 columns by 1072 slices. Your y is 357 so that's why it's first and you have 357 rows. When the workspace says n*m*s, the first number is the number of rows, which is y, not x.
x = 1:532; % Columns
y = 1 : 357; % Rows
z = 1 : 1072; % Slices
[X,Y,Z]=meshgrid(x, y, z);
[rows, columns, slices] = size(X)
