Archer turned 3 in January. He weighs 32 lbs and is 37" tall. He wears a size 3-4 in Zara and Gap clothes, so our clothes fit very similarly. Note: He is wearing a size 4T in the shorts in all the photos.
Here I'm comparing our tanks size 1-2 (formerly called 2T), 3-4 (4T), 5-6 (6T)
We changed what we call our sizing this season as we found some people would wonder where their childs size is and if we didn't carry it. Since we always told people to size up if between our even numbered sizes, we just changed the numbers to reflect this formula. Sizing is such a tricky thing and it can be stressful trying to buy without seeing it on. Here you can see that there is not a lot of difference between the 1-2 and the 3-4. Archer actually has both sizes hanging in his closet and I interchange them. Size 3-4 is the best fit for him at this size though.
Here I'm comparing our size 1-2 (previous called size 2T) tank vs t-shirt. You will see the t-shirts fit a little larger. The tank is pretty skin tight on him though not too small.
Here Archer is wearing the 3-4. This is the size I would likely recommend to most people with a 3 or 4 year old. This size was previously called 4T. Even though our tanks do run smaller than the t-shirts most kids still end up wearing the same size in both.
Ok the chocolate chips kicked in! Here is Arch in our size 5-6 (previously called 6T). But you can see it's much too big on him. He will likely wear a size 3-4 for the next 2 summers.
Hope this was helpful!