Your party is level 1. At this level, it's fairly common for an enemy to fall in a single hit. Even a wizard with a quarterstaff is only 1 BAB rank and 4 hp difference from a fighter, and their familiar probably has the highest AC in the party.
My advice is not to worry about balancing encounters until the group is about level 3-4. Going too strong at lower levels runs an extremely high risk of a TPK unless you and your players are very familiar with each other's styles. Run them through a few easy encounters to let them build a roleplaying story together, then challenge them with something tougher.
Still, if you want to create more of a challenge, use environmental hazards and give the opposing team the advantage.
For example, my own homebrew campaign began at level 1 with five players. The scenario was to track down a group of orc bandits hiding in a swamp. A series of easy random encounters to get to the orc hideout ensues, starring a goblin dog and a pair of stirges. The group steamrolls them, building their confidence and introducing their playstyle at the same time to everyone at the table.
Then things get tougher. The next encounter involves a lizardfolk and a hidden patch of quicksand. By now I've got a read on the group and know how they'll react. Thus, I bait the party's melee fighter by having the lizardfolk lob javelins at him from cover. He takes the bait and charges straight into the quicksand. This locks down two characters in the group as the strongest melee fighter is badly failing to stay above the surface due to armor check penalties from his banded mail, necessitating a second player to aid him and leaving only the three weaker members to fight the lizardfolk. They survive but are more cautious now.
Finally they attempt to cross a river, only to encounter a pair of Grindylow (water octo-goblins) lurking under the surface. Nobody beats their Stealth, so they get a surprise round to attack while the group are crossing at the deepest part. Cue water combat rules and drowning rules as well as ranged attack rules against underwater creatures. In short, the party was screwed. Luckily they had two characters with mounts, meaning they could attack from their mounted position and managed to hit and one-shot the monsters before their ally drowned.
In both of the more difficult scenarios, the CR of the encounter was a straight level 1 fight, but unlike the previous encounters the environment played a big part in their difficulty. Other examples I've used in this game include a kobold infested mine with a guard tower that was surrounded by a field of spikes. Improved cover for the kobolds shooting out of the guard tower and no way to close in to engage in melee makes for a difficult fight. In this case, my players used creative thinking to have their kineticist take out one of the tower's support legs from range, toppling the tower and crushing the kobolds inside.
Also don't forget that gear and equipment on a monster isn't just loot for the players. A flask of alchemist's fire is a nasty weapon in the hands of a goblin at low levels. A smokestick can screw any ranged attackers. A scroll of vanish can turn a sorcerer/rogue duo into a lethal team.
So a couple important things.
The game is actually balanced around 4 players, that are not particularly optimized, use a 15 point buy and are composed roughly of the classic 4 party structure. That means in combat you have the guy good at fighting (fighter), the guy who sort of fights and has skills (rogue), the guy who sort of fights and casts divine spells (cleric) and the guy who casts arcane spells (wizards).
However pathfinder has transitioned a long way from that classic party, and you have keep in mind, if you have a party of 4 players all good at fighting, then that is way more offensive combat then is normally expected. IE a party of a combat druid, a summoner, a warpriest, and dawnflower dervish bard can fight a lot more then the classic 4 can.
Add in additional people and optimization and you again have skewed the numbers.
My rule of thumb Is as follows, for each of the following things, raise the APL by 1.
1. Each person over 4 (yes I know the game says 4-5, but there is a difference between 4 players and 5 players and its silly to just assume its the same).
2. If your stat generation is more generous then a 15 point buy.
3. For each 2 characters that are particularly optimized.
4. If more then half of your party is very good at combat.
The key though, is don't increase CRs by enhancing monsters/enemies, but by adding more.
My guess is you probably hit each of these with your party, so a 6 person party, with better then 15 point buy stats, 2 characters that are optimized and is combat heavy, I would peg at +4 or 5 apl, and add additional monsters to encounters to beef up encounters accordingly. So if I had an ecounter with 4 wolves normally a CR 5 encounter, I would probably make it around 12-16 wolves bringing it up to CR 8 or CR 9 (this takes a bit of judgement based on what your party seems capable of, start by adding 3 to CR then go to 4, then 5 until you hit a comfortable spot.
Remember this is more art then science. CR is not a perfect measure of how much a given encounter will threaten your specific party, but in time you will learn to better figure that out for yourself.