"I'm 92 kilos now, which is just about where I want to be," he said. "When you're heavier, the turning is the difficult bit, that's when you feel it.
"Running in a straight line it's not too bad, because once the weight is moving, it's moving."
England will face ex-boss Eddie Jones's Japan in Tokyo on June 22, before two Tests in New Zealand in July.
Head coach Steve Borthwick prizes Sleightholme's game-breaking and try-scoring abilities, and the Saints star feels he has now struck the right balance.
"It's a fine line between being too big and being right in a good spot," said Sleightholme.
"I was too bulked up and through the year found where I needed to sit with my body and then got into a rhythm."
