Lucy's MP should not be the same as her spirits. It says here that the Spirit King gave transfered Aquarius's magical power to Lucy, meaning that Aquarius had her own reserve of MP.
You're trying to transform an exception into the rule.
They have their own MP in the Celestial World, there they wield their full power all the time, but in the mortal world their power is bounded to Lucy's MP, she puts a limit in their maximum MP. They could not be more powerful than the person opening the gates, that's also why she could not summon more than one at the beginning of the story since she is too weak to meet the minimum MP required to summon more than one spirit and why when she unlocked second origin she could not only bring more spirits at a time but their powers were stronger than before.
An analogy that could be made is that Celestial Spirits area like a summon on a RPG game, a Level 1 summoner would summon summoning a wolf would summon a familiar that doesn't have much HP, much MP and much Attack, a level 99 summoner, summoning the same familiar would summon him with stats far stronger. You can say that the level 99 summoner is summoning the familiar at its full power, while the power of the familiar of the level 1 summoner is tied to the power of its caster.
Summoning the Spirit King is the exception to this rule, that's why she have to sacrifice one of her golden keys in order to be able to do it. This requirement to sacrifice a key probably bypasses the requirements necessary for the spirit to come to the mortal world, also when Lucy summoned the Spirit King, she was already exhausted of her own MP, which means that when the Spirit King was summoned he in fact it seems that he has transferred Aquarius own MP in order fill Lucy's magical container for her to fight alongside him. Even so, it seems that even if he came with his full powers and on his own, it still has something to do with the amount of power Lucy could wield, since it still have a time limit for him to be in the mortal world.
I still think the way I explained it fits the bill. Lucy's spirits get stronger as Lucy gets stronger because Lucy adds her own magical power to them. Naturally, the more power Lucy can afford to give her spirits, the stronger they become. Also naturally, spirits who are stronger would require Lucy more power to summon. A spirit like Gemini might have very little magical power on their own, so they tax their magical power from their owner. This effectively makes Gemini's limit whatever their owner's power level is. A spirit like the Celestial Spirit King has more magical power than any Celestial Spirit Mage, they can do stuff that the spirit mage themselves can't.
Star Dress lets Lucy borrow her spirits' magical power. It probably lets Lucy convert her own magical power to fuel the star dresses abilities. Effectively, it makes Lucy less of a target, and lets her fight more hands-on. I do think certain star dresses has the potential to double Lucy's power level. For example, a CSK star dress should more than double Lucy's power. However, there's nothing that implies that a Virgo or Taurus Star Dress would achieve the same effect. Taurus and Virgo aren't stronger than Lucy as an individual. Even if you were to add all of their power to Lucy, it still wouldn't double it. Which means that if Lucy's power is doubling, it would be for no apparent reason.
That's makes no sense, MP is always was always described as a container that can be filled with magical particles and that the power of a mage is defined by the size of his container, a mage could increase the size of his container by training, and sometimes he could use concentrated forms of eternano particles in order to wield powers beyond their normal limits, like when Natsu ate a piece of Etherion charged Lacrima or when Wendy ingested Face's air with a higher concentration of eternano particle.
There is no indication whatsoever that summoning a Spirit could increase the amount of MP a Celestial Spirit Mage could wield, but there are a lot of instances where it was implied that the strength of a spirit is tied to the amount of magical power the mage has and that there is a limit to how much time and how much of their own MP they could use in earthland without relying on the power of their casters. For instance in chapter 46 Lucy clearly states that the power her spirits could wield when summoned are dependent on the amount of power she has which in turn depends on the amount of training she does, she even states that if she trains enough she she could summon her spirits with powers stronger than Natsu's, later in chapter 72 Loki states that a Celestial Spirit could not survive long outside the spirit world and that they depend on their summoners power in order to survive here, if they stay without being summoned or without relying on their summoner's power they would eventually die, and in chapters 114 and 146 it is shown that if Lucy MP reaches zero only stronger spirits like Leo and Virgo could summon themselves without relying on Lucy's power.
Outside the Manga, in the Anime version also there is an arc where Ophiucus uses an item to control the other spirits and they have to fight them in the Spirit World, the spirits gave a very hard time fighting against the likes of Natsu, Erza, Mirajane and Elfman with their second origin unlocked. Which means that there, with their full MP they are far stronger than what was shown during Lucy's fights.
Many people already ahead of Lucy even with her star dress and 4 spirits she still can't do much but it will surely increase her potenial in a fight.
