As suggested by @Anthony Aslangul there is limitation on xls when compared to xlsx. Here are the few key differences
| Feature | XLSX (Excel 2007 and later) | XLS (Excel 97-2003) |
|---|---|---|
| Rows per worksheet | 1,048,576 | 65,536 |
| Columns per worksheet | 16,384 | 256 |
| File size | Limited by system memory | 2 GB |
| Number of colors | 16 million (32-bit) | 56 |
| Unique cell formats | 65,490 | 4,000 |
| Maximum formula length | 8,192 characters | 1,024 characters |
Here is some of the links for referrence