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The apparent contradiction between small-scale source regions of, and\udlarge-scale coronal response to, coronal mass ejections (CMEs) has been a\udlong-standing puzzle. For some, CMEs are considered to be inherently\udlarge-scale events – eruptions in which a number of flux systems participate\udin an unspecified manner, while others consider magnetic reconnection in\udspecial global topologies to be responsible for the large-scale response of\udthe lower corona to CME events. Some of these ideas may indeed be correct in\udspecific cases. However, what is the key element which makes CMEs\udlarge-scale? Observations show that the extent of the coronal disturbance\udmatches the angular width of the CME – an important clue, which does not\udfeature strongly in any of the above suggestions. We review observational\udevidence for the large-scale nature of CME source regions and find them\udlacking. Then we compare different ideas regarding how CMEs evolve to\udbecome large-scale. The large-scale magnetic topology plays an important role\udin this process. There is amounting evidence, however, that the key process\udis magnetic reconnection between the CME and other magnetic structures.\udWe outline a CME evolution model, which is able to account for all the key\udobservational signatures of large-scale CMEs and presents a clear picture\udhow large portions of the Sun become constituents of the CME. In this\udmodel reconnection is driven by the expansion of the CME core resulting\udfrom an over-pressure relative to the pressure in the CME's surroundings.\udThis implies that the extent of the lower coronal signatures match the final\udangular width of the CME

Why are CMEs large-scale coronal events: nature or nurture?