As long as you're taking testosterone in substantial amounts, Clomid will do nothing for testicular size.
The way it works is this: The testes are stimulated by LH, which signals them to produce testosterone.
Either high androgen levels or various estrogen levels will reduce or shut down LH production. With estrogen, there's substantial inhibition even with moderate levels, increasing to total shutdown as levels go outside the normal range.
Post-cycle, androgen levels are at most normal or even low, but estrogen levels will still be where there's partial inhibition of LH.
Clomid blocks that inhibition. So, more LH.
While you're giving yourself relatively high androgen levels with your injections, your hypothalamus is (illustratively speaking) getting an "OK" with regard to estrogen signaling thanks to the Clomid, but a "NO" with regard to androgen signaling. So, no, or negligible, LH increase.
HCG acts like LH and so works regardless of androgen levels.