The phrase "big spike" means a sudden and significant increase or surge in something, like a sharp rise in numbers or a sudden burst of energy or activity. Full definition
Avoid sugary cereals and pastries as they will cause too big a spike in blood glucose and insulin and will leave you feeling hungry in an hour or so. (fitnessandpower.com)
At breakfast, same as before, big spike with the bread; small spike with lentils. (nutritionfacts.org)