I'm not sure what you are implying, but I'll go ahead by explaining how a Pokemon's stats are determined. A Pokemon's stat is determined by a variety of things... namely BASE STATS, IVs, EVs and nature.
Let's take a look. So let's say you wanted to train a Lucario (just picking that Pokemon for your namesake). These are Lucario's base stats:
70 HP
110 Attack
70 Defense
115 Special Attack
70 Special Defense
90 Speed
I'm only posting the stats out of my mind, so I can be wrong here... but just for the sake of explanation, we'll get through this.
For now, we will assume neutral nature for this Pokemon. With those base stats, Lucario can reach a maximum of the following with 31 IVs in each stats and 252 EVs in each:
70 HP = 344 HP
110 Attack = 319 Attack
70 Defense = 239 Defense
115 Special Attack = 329 Special Attack
70 Special Defense = 239 Special Defense
90 Speed = 279 Speed
Remember this is assuming neutral nature, 31 IVs in each stats, and 252-255 EVs in each stats (however, having 255 in EVERY stats is impossible without cheating, but we'll get to that later). But how did I figure that out? Okay, just remember that a base 100 Stat in something with NEUTRAL nature, 31 IVs and 252-255 EVs in it is 299. Each base stat is equal to 2 absolute stat points, so something with 101 base stat in a field would have with the condition listed above 301 stat, while something with base 99 base stat would have in that field 297 stat. These are for Attack/Defense/Special Attack/Special Defense/Speed stats. HP stat works differently. Just remember base stat HP of 100 will have a maximum of 404 HP (with 31 IVs and 252-255 EVs)... and count 2 base stat down from that. Every 5 base stat = 10 absolute stat points... is all I'm going to say.
Confusing? Yeah I'll bet it is. What are EVs and IVs and nature? EVs are controllable by the trainer while IVs and nature are... not controllable at all by the trainer aside from hacking. Let's get to it...
For the IVs, a Pokemon can have a value of 0-31 IVs in each stat. 1 IV is equal to 1 absolute stat point, so having 28 IV in attack will give you 28 extra attack points. IV stands for individual value, and that is why your Lucario looks better than your friend's because yours has 28 IV in Attack while your friend's has only 6... but then again, your friend has 19 IV in speed while yours only has 2 in speed. Anyways, you get the point. IVs are basically randomized on each Pokemon you have and are basically uncontrollable. However, if you breed two Pokemon, the baby will have a few random IVs selected from each parent, which is SOMEWHAT controllable, but the ones chosen are definitely not.
EVs? Okay... here's the deal with EVs. For every Pokemon you fight, depending on what you fight, you gain EFFORT VALUES. Every stat can have a maximum of 255 EV points each... while you can have an absolute total of 510 EV points max, meaning it is only possible to "max out" 2 stats, a feature I quite like due to more customization and less stall wars like GSC. Unlike IVs where a single IV becomes one stat point, you need FOUR EVs to make 1 absolute stat point. Because of this, the maximum EV you ever want in a stat is 252, because 252 is divisible by 4 while 255 is not. Since 510 is the absolute max and it is 508 that is divisible by 4, the remaining 2 EVs are always wasted because they do not count at all. Need to know which Pokemon gives which EVs? Stuff like Zubat gives you 1 speed EV while a Geodude gives you 1 Defense EV... a Golbat gives you 2 Speed EV... it is quite complicated so look into another source for that, like here. No difference b/w Gen III and Gen IV EV yields... except for one I know... Chansey and Blissey gave 1 and 2 HP EVs respectively in Gen III, but now thanks to the addition of Happiny, Chansey gives 2 HP EVs and Blissey gives 3 HP EVs.
Nature... well, you can see a Pokemon like your Lucario has Lonely Nature or something. If your Lucario is Lonely, it gains 10% more attack power, but 10% less defense. All natures except for Bashful, Docile, Hardy, Quirky, and Serious raises one stat by 10% and another stat is decreased by 10%. So which stat does which? There are many resources online that can tell you which is which. For example, click me.
So rare candies do nothing but level up your Pokemon... except the only reason why it is said to make your Pokemon weaker than others is that you're leveling a Pokemon STRAIGHT without EV training it. However, if you EV trained it to its fullest, then you can rare candy all you want without caring about its stats "lowering" or anything since they really are not.
Hmmm... I may have to reword all this later because it's more of a stream of consciousness than anything else, so if it is confusing you, sorry