I am specifically referencing that when I look at maps, I often see that phenomenon happening.
For example:
The Roman Empire often just held control of the coast of North Africa, but muslims seem to go well beyond that. Also and especially true for Morrocco where different rulers often went well beyond the border, you'd see marked as southern border of the Roman Empire. The Arabian Peninsula is also a good example. As well as Persia, and the steppe areas east of the caspian sea. Or north of the caucasian mountains.
Or crimea. Before the byzantine Empire was replaced by the Ottomans it held crimea, but often not even the whole island, but just the southern coastal strip. The Ottomans on the other hand often went well beyond that and held huge areas of modern ukraine.
Maps are a subjective representation. The Roman Empire only controlled the coast and the Caliphate controlled the interior of North Africa because the mapmakers decided to color in that area differently, not necessarily because the extent of their control was hugely different. In some cases it was, but it's important to remember that maps can be misleading, and are often based on flimsy and inconsistent logic.
To address the Ottoman/Crimean issue - The Ottoman Empire, like the Byzantines, only maintained direct control over the southern coastline of the Crimean peninsula, where they maintained garrisons in towns like Kaffa (Kefe). The rest of the peninsula was ruled by the Khanate of Crimea, which was a vassal state of the Ottomans from 1475 onward. The Crimean Khans themselves exerted control over the steppe lands of modern Ukraine by maintaining the loyalty of the nomadic groups living there. However, on maps this whole region is sometimes simply marked as an integral part of the Ottoman Empire.