Hang on to your helmets soldiers because there is going to be a lot of common sense reasoning and very lightly researched topics here.

There is little data except anecdotal evidence regarding the first part of you question, however, it is also non-controversial to assert that size in general is an advantage in melee combat. All other things equal, the larger fighter has an edge in most situations. On average he will have better reach, more muscle mass and more mass in general, effectively allowing him to reach further, hit harder and larger absorbtion zones. If we take a wider historical perspective we can also assert with some certainty that physical size generally corresponds with genetic health and good access to food, both of which will be generally advantageous in combat. So can we prove that physically bigger and stronger soldiers had a greater expectancy of survival? No, probably not, but we can assume that it was the case as it is generally consistent with other factors.

What about when we started shooting each other though? Well, here's were it gets interesting. You'd think a larger soldier would be an easier target, right? It's undeniable true. However, that does not mean his chances of survival are worse. In fact, there is good evidence to assume that the exact opposite is true. Why? For several reasons.

First and foremost, getting hit is not the same as getting killed. The vital organs do not grow linear to general body size and given that most soldiers were not enormously larger than the average we can actually assume that their vital organs were about the same size. What this means is basically that while your chances of getting shot may be marginally higher, your chances of getting shot in a vital organ are actually about the same and your chances of recieving a non-mortal wound (possibly getting you out of the war) are actually higher. Added to this, your greater size may actually means that you're more likely to survive a shot that would have killed a smaller man simply because you have more non-vital tissue in the way and simply a bigger body to absorb the damage.

Second, size generally corresponds with genetic health and general fitness. While a larger soldier is a bigger target he is also statistically faster, stronger and generally fitter, all things that generally make soldiers a harder target in combat. Again, this advantage may well out-weigh the miniscule increase in size of your vital organs.

Third, size has a generally positive correlation with intelligence (again, this may well be just another result of overall genetic health and nurture). A more intelligent soldier may be more likely to achieve an NCO position or some other position that isn't a rifleman. Basically any other position has a better survival rate than the rifleman and thus give your larger soldier a better chance of survival.

Fourth, bullets aren't generally what kills you, it's artillery. Bullets only account for somewhere between 20-40% of battlefield casualties, artillery is the real killer with anywhere from 50-80% depending on theater and year. In the case of artillery your are again, less likely to be hit if you're not an infantryman and probably more likely to survive be more mass you have and the better your overall health is, both of which correlate with size.

So... are larger soldiers more likely to survive a war? Actually yes. If we look at WW1 where we actually have fairly reliable figures of at least height of soldiers we see that surviving soldiers are actually significantly taller than their non-surviving brothers in arms. For British soldiers it was almost a full inch in WW1 (2,37 cm to be precies), far too much and far too large a sample size to be mere coincidence.

TL;DR - It is highly likely that size was and still is a significant advantage for soldiers but there is no one mechanism behind it.

Sources: Records of all British soldiers, their fate and their height are available in the national archives.

Average size of surviving British soldiers and corresponding figures for non-survivors from: Big and tall soldiers are more likely to survive battle: a possible explanation for the ‘returning soldier effect’ on the secondary sex ratio, Satoshi Kanazawa - Human Reproduction, Volume 22, Issue 11, November 2007