I have been reading about several big 18th century wars recently, namely the War of the Austrian Sucession, the Seven Years War, the War of the Spanish Succession and the Great Northern War. Each of these conflicts spanned many years and involved major powers of its time. However death toll estimates I can glean from this are low only the seven years war cracks a million deaths. Later european wars of similar involvement of powers like the Napoleonic wars and earlier great wars like the 30years war had larger death tolls. Is this an artifact of estimation errors or were these wars really much less deadly? If so why?
During the 18th century warfare actually declines in frequency, compared to the 17th century which had 95 years of warfare, the 18th century only experienced 78 years of warfare. However the difference became that wars became more intense and harsher in respect to the environment and infrastructure thus you see an increasing severity and length of wars. I'm not sure what your figures point to as a figure but Levy in his book War in the Modern Great Power System, has estimated a death toll of just 3,758,300 for the period 1700-1802.
I think the shortfall in the death toll is down to the military evolution that took place over the course of the 17th and 18th centuries. The new tactics and strategies that were introduced such as those adopted by Maurice of Nassau and Gustav Adolphus in the 17th century are now adopted by all major European nations and even though more troops are required to carry out the more ambitious strategies the tactics are created to lower the death count. The tactics require a specialization and training of troops rather than just untrained mercenaries that had made the bulk of the 17th century armies. Moreover there is a reluctance to partake in pitched battles and a more general reluctance from the major European nations to pressure a fellow European nation into submission as this would have too great an impact on the balance of power within Europe. The most glaring example of this would be the treaty between the French and the British after the French and Indian War, were Britain returns France territory in South America (to be fair they also had mercantile reasons for this as well though). It's also important to note that the biggest territorial gains in the 18th century were ultimately won through diplomatic negotiations rather than militarily overpowering the opposition. If you want to read more on the military evolution of the 17th and 18th centuries I would recommend any of the following (mhra referencing):
Anderson, M. S., and Geoffrey Best, War and Society in Europe of the Old Regime 1618-1789 (Stroud: Sutton, 1998).
Black, Jeremy, War and the world, (Yale University Press, 2008)
Parker, Geoffrey, The Military Revolution, (Cambridge: Cambridge University Press, 2011)
Parker, Geoffrey ‘The Military Revolution 1550-1660 - A Myth? Journal of Modern History 48:2 (1976): 195-214
Rogers, J Clifford, The Military revolution debate: readings on the military transformation of early modern Europe, (Boulder, Colo: Westview Pres 1995)
One last reason may have been that due to the increasing scale of warfare, military aims had to be thought of on a more holistic scale - there is no point winning a war in only one theatre, you have to win in all of the theatres of war to ensure/claim a victory. So in effect military aims and pursuits were scaled back in a micro sense to achieve victory on a macro scale.