Tonnage is the force applied to clamp the mold together. The bigger the mold the more tonnage you need to hold the mold together and prevent the plastic from running out of the shut lines ( flash). This machine is too small to make a standard builders buckets, a limiting factor for the part size is the distance between the tie bars. If the bars are too close together you can't get the mold into position. The machine would need to open up wide enough to eject the bucket.

Injection pressure is another factor, bigger machines can squirt more plastic before the liquid plastic begins to solidify. If you have thin walled parts this is even more important as the thin areas will cool faster and block any more plastic getting in to fill the mold.