From the documentation on xlswrite:
A — Data to write matrix | cell array
Data to write, specified as a two-dimensional numeric or character array, or, if each cell contains a single element, a cell array.
If A is a cell array containing something other than a scalar numeric or a string, then xlswrite silently leaves the corresponding cell in the spreadsheet empty.
The maximum size of array A depends on the associated Excel version. For more information on Excel specifications and limits, see the Excel help.
Now a little Google yields for e.g. Excel 2010:
Worksheet size 1,048,576 rows by 16,384 columns
For Excel 2003 it is much less. Since your matrix is "huge" you're likely exceeding the column limit.
You could try csvwrite or dlmwrite to store your data, a simple text file should not have any limits. However, you won't be able to import or open those files in Excel then.
If that doesn't solve your question, then post more details, like the Matlab and Excel Versions you are using, the dimensions of your matrix, and the error message!