Personnel is much more apples to apples. Comparing spending between two nations muddies the waters.
I assume you're commenting on US spending, but consider that other countries don't have to pay US wages and benefits to it's soldiers, contractors, and manufacturers. If we could buy our weapons from India, recruit our soldiers from China, and build ships in S.Korea our defense budget would be much smaller without having to ration a single fighterjet.
Did you know that we spend almost 4X as much on social programs than we do on the military today?
You know how much Hess countries spend on defense? The US could buy enough bullets to kill everyone in the world 10 times over and miss 50% of the time.
How is it a shit metric? They said the largest army. What do you think an army is? It's soldiers, it's people. The graphic wasn't what's the largest Arsenal or the largest navy or the largest Air Force Etc..
Now, this is based on slightly outdated statistics but let's roll with it for a second.
Vietnam's Reserve, according to this chart, is 5M give or take.
Vietnam's 2018 Defense Budget was 5.5B USD, give or take.
If somehow we spent that entire budget on the Reserve, we would end up with an expenditure of $1,100USD per Reserve Soldier.
Counting Uniforms, Food, Weapons, and Ammunition for training, how much quality training do you think you can accomplish for $1,100USD per Soldier per year? Even taking into account currency conversion, $2.3M VND per month isn't a lot when you're talking military budgets. At that point, we should just count draft card holders as "Reserves" for the US.
An army is a tool for accomplishing goals in defence from and enactment of violence on actors of significant capability. Personnel are just one of the few raw resources said tool consumes. Conveniently said personnel can be used in place of other resources in this calculus but usually at significant inefficiencies. This can even mean that militaries with inflated personnel may even indicate that said military is lacking in other critical areas which must be plugged with disposable personel ie look at the Russian military and its meat wave attacks.
You can't use numbers of people to make comparisons between countries because they are misleading. Some countries use their soldiers for construction work (China) or have whole industries owned by the military (Iran). A person working on a defense industry assembly line isn't a member of the military in most countries.
The Islamic Revolutionary Guards were put in charge of creating what is today known as the Iranian military industry. Under their command, Iran's military industry was enormously expanded
Using numbers of personnel to compare China or Iran to a country like Belgium would make Belgium look like a pushover. Belgium has a tiny military but uses it's location in Europe to ensure security through diplomacy and membership in NATO.
I promise you would be harder to invade the headquarters of NATO than either of those countries.