Some tasks are just too big or uncomfortable to do that the feel intimidating. But those tasks need to get done too. So which one is better to start with.
Eating the frog refers to a quote by Mark Twain. He said that starting with the task you are least motivated to do makes the rest of the tasks easy. If you have worst behind you there is nothing stopping you anymore.
On the other hand, if you start doing smaller tasks you might end up not having the time to eat the whole elephant. Getting those big time consuming tasks out of the way reduces your risk of going overtime. Or having to leave a task half done.
Some frogs might seem like elephants not because of their size, but because how hard they appear. Likewise some big tasks may start turning green just because of the sheer amount of work they require, no matter how easy the task itself would be.
Maybe we should follow Mr Twain's advice to the end as he continues with "if you need to eat two frogs, it's better to eat the bigger one first.