You shouldn't be stranding the yarn across the back. Doing intarsia means those areas are worked, usually with a bobbin but you can use a separate skein, and the result is blocks of color...no strands. By stranding the 'floats' are pulling across, creating tension.
ETA: some good pics here
Not sure what the blue/pink(?) image is you posted but here is site with both RS and WS of heart done in intarsia. Note the absence of the stranding.
In a small area, where using a sep skein or bobbin isn't justified, and you feel the need to strand, floats should be worked into the back of the foreground color stitch every few sts so the floats are minimal and the tension remains the same as the main color. (You have long, tight floats on the back of portions of the color block image.)