Complex sentence: There are no records about the size of the two armies, but the Bavarians were so confident in their superior numbers that they split their army in three groups, implying that they thought each of the three groups was bigger than the whole of the Hungarian army.
Split into simpler sentences: There are no records about the size of the two armies, but the Bavarians were so confident in their superior numbers that they split their army in three groups. This shows they thought each of the three groups was bigger than the whole of the Hungarian army.