Insulin will always make everyone get bigger when used proplery...IF (big if), you're eating enough cals and overall carbs so that the body not only has enough cals to support it's current bodyweight, but also has enough carbs to fill glycogen stores beyond normal limits.
Using insulin during a caloric deficit will signifcanty reduce your overall ability to fill out your musculature, as the body will use the carbs it needs for energy before keeping it stored as glycogen.
You also mention you are consuming no fat at the time the slin is active, yet you are consuming NO other carbs with your other meals. Being that a single dose if Humulin R will only stay active about 5 hours (it is mostly gone by then), that means you are eating zero carb diet the majority of the day.
Now, I don't know what your metabolic needs are or how many carbs you require to maxmimize glycogen stores, but I will tell youi it is impossible to attain maximum muscle fullness when eating zero carbs for the maority of the day. Even if you took in about 200 grams of carbs when the slin was active at your pre/intra/post workout meals or shakes, but ate no other carbs the rest of the day, you should not expect great results with any insulin program.
You will already be burning offf some of those carbs during the wporkout itself...and any carbs your do store during that period will be taken right back out fo storage and used up during the rest of the day when you aren;t eating any carbs. 200 grams of carbs is a very low amount of carbs to eat for an entire day. Most guys will need between 2-3 grams of carbs per pound of bodyweight (on average) if they want to gain mass and maximize glycogen stores.
Insulin will increase your bodyls ability to store glycogen, making your muscles swell and look bigger, but if you aren't eating enough carbs to fill up yout glycogen stores to bein with, it won't matter how much insulin you use....your results will alays be impaired.
As far as 20 lbs in 4 weeks goes, that is NOT common with just a pre-workout protocol and I would not have said that, as I know it is not true. Now, people "can" gain 20 lbs in 30 days from insulin IF the circumstances are right, now with the protocol you are talking about, I would day 10 lbs is probably average in the first 2 weeks. By that time, you will have increased your muscle fullness about as much as you're going to with that protocol, assuming everything else remains the same.
With insulin, the weight gains which take place in first few weeks are always attributable to an increase in muscle fullness..NOT muscle growth. After fullness has been maxed out, any further lean weight gains caused by insulin will be muscle tissue, but that is a much slower prpocess than increasing muscle fullness, which is why the weight gains slow down so much after the first few weeks. Insulin causes growth in a few ways, such as activating genes directly related to protein synthesis...by indirectly increasing protein synthesis through increased muscular volumization...and by directly increasing protein synthesis through enhanced nutrient transport to the muscles.
If you haven't gained any fullness in the first week from the current program you are following, there is some aspect() of your program which is deficient.