Unfortunately, you can't catch 'em all.

While you may be able to encapsulate smaller monsters, the Targets section of the spellcasting rules (PHB p. 204, BR p. 102) state

"To target something, you must have a clear path to it, so it can't be behind total cover."

Since you target a point - you must be able to see that point, and as framed, you cannot currently see inside the Tarrasque.

But let's say you could, for whatever reason. The paragraph continues with,

"If you place an area of effect at a point that you can't see and an obstruction, such as a wall is between you and that point, the point of origin comes into being on the near side of that obstruction."

So while Wall of Force allows the caster to decide whether or not to shunt the creature forward, it ensures that the dome comes into play on the outside of the Tarrasque.

Seamus asked, "Wait... Can't you target a point near the Tarrasque that you can see, but still creates a sphere that intersects the Tarrasque?"

The rules for Area of Effect can be found in the next section of the rules. The rules state,

"A spell's effect extends in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area."

Rules as Written are Unclear

The Force Cage spell states (PHB, pp243)

When you cast the spell, any creature that is completely inside the cage's area is trapped.

And then separately:

Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area.

Given that these are are two separate sentences, the "too large to fit" clause may or may not be directly tied to the "when you cast the spell" clause. If it applies to both at time of casting as well as during the life of the spell then the creature is shunted (if it is two sizes larger than the cage - see squeezing rule.) If it only applies to the cage creation, then it raises the issue of a creature being in too small a space (see below.)

If those two sentences are supposed to be directly related, it changes the interpretation and then allows for use of Force Cage to autokill something. There are no real rules for this, and this question on polymorph may be related with regard to too small a space. See the Concerns section below for more information.

Squeezing into a smaller space

On the chance that the polymorphed creature is just one size smaller then the creature itself, the Squeezing rules would come into play (PHB, pp192)

A creature can squeeze through a space that is large enough for a creature one size smaller than it...It has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.

Concerns

There are two concerns that you should consider when determining use of this at your table:

1. If you choose to read the Forcecage text as not applicable post-casting, then you have created a neat effect to instantly kill a creature. This could become very powerful if you have a group that is able to do this. However, this is also using a 7th level spell slot (along with a 3rd) - and may not be as unbalanced as it seems. This is similar, but not identical, to the idea of polymorphing into a tiny creature to be eaten or go into a larger creature and then end the polymorph so you "burst out" and kill the creature. There are no RAW rules around this and allowing tactics like this potentially creates a slippery slope (although not as great when you consider the 7th level spell cost for Force Cage.)

2. If you choose to go with the interpretation of being shunted out, you are risking devaluing the Forcecage. The odds of this happening are small as it is unlikely to be used against the party by an enemy.