If you're five of the ten toons in a 10-man BG...
It almost doesn't matter what you have or how well geared you are.
Your team almost never wins, with 50% of the group always in one place.
As 50% of the battleground, your gearing and skill means a massive amount.
You can win WSG/Gilneas, if the other five guys are good enough to hold one point or return a flag... but usually these are losses; I've had WSG matches where all ten of the other side were easy to kill with my five, but I couldn't both guard my flag/intercept their carrier and capture their flag with the useless teammates who contributed nothing.

If you're five of the fifteen toons in a 15-man BG...
You comprise 33% of the battleground, so your gearing and skill are less important.
But with at least full honor gear, you can make a positive impact in many of the BGs.
EotS favors 5-boxing, where you just run to whatever they hold and take it, repeat ad nauseum.
AB is a little harder, because you have to stay in place for a time until the flag caps... which negates the advantage of being able to blow up almost anything once you're geared; I've had ABs where my five can consistently kill their 15, but our 15 cannot hold more bases then their 15.
SotA is crappy boxed, because on offense your team is split which often leads to being stuck on the beach... an inherent disadvantage to boxing this BG.



If you're five of forty players in a 40-man BG.
You comprise 12.5% of the battleground, so your gearing and skill are the least important.
You have the ability to blow up large groups, and if you do it holding an objective or recapturing one that can contribute to a win.
As a small portion of the BG, when you're starting out you want to be here so your team still has a chance to win.
When you have a lot of gearing, this is still the best BGs to be in as a 5-boxer, because having 12.5% of the team always in one place is not an inherent disadvantage.

When I refer to battlegrounds as a 5-boxer, it is almost always referring to AVs and IoCs...
Almost anything is going to work, because it is largely a PvE event except for mass numbers vs mass numbers at certain objectives along the way.
When it comes down to a 10-man, you can win 100% of the battles and never lose a toon, be the highest in damage/heals/honor-kills/killing-blows/etc and still consistently lose; that is the reality of the smaller BGs.