Bigger Stacks is the 1st upgrade of Path 1 for the Spike Factory in BTD6. It increases the number of spikes produced by the Spike Factory from 5 to 10, equivalent to +5 pierce. There is a special interaction that increases Perma-Spike spikes from 50 to 80, for +30 pierce.
Bigger Stacks essentially doubles the number of spikes for the same price as the base Spike Factory itself, with the only exception being Perma-Spike, which is only a +60% pierce increase. The Bigger Stacks upgrade is very cost-effective in combination with crosspaths, and buying a 1-0-0 is more cost-effective than buying two 0-0-0 Spike Factories.
Bigger Stacks is good in combination with other upgrades, so buying this upgrade will become useful when crosspathed with another upgrade, especially with Path 2 upgrades.
Generally speaking, this upgrade is the best crosspath with Upper Path 2, specifically for Spike Storm and Carpet of Spikes, as not only is White Hot Spikes is accessible, it provides a reliable source of higher overall DPS, in contrast to buying Smart Spikes, where higher ability spike DPS is limited to the start-of-round stages.
If not planning to buy further crosspaths with Bigger Stacks, it's better off buying a second 0-0-0 Spike Factory. That way, for a similar price, the overall spike production can replenish at a faster rate. Not only that, but with two Spike Factories, spike production will be less swayed by RNG if bloons end up reaching near the Spike Factories.
Two 0-0-0 Spike Factories cost $2000 on Medium, while one 1-0-0 Spike Factory costs $1800 on Medium.
Bigger Stacks in itself has been nerfed since it was first released, with not only price nerfs to the upgrade itself but also to the base Spike Factory price and decreased base lifespan of the spike piles. On the flip side, the Bigger Stacks crosspath with Perma-Spike has been buffed considerably to balance out with the more popular Even Faster Production crosspath.
1-0-5 Perma-Spike pierce increased per spike (60 → 70).
Base projectile lifespan reduced from 70s to 50s.
Cost of Bigger Stacks increased ($600 → $800).
Affected by Spike Factory crosspath speed changes.
1-0-5 Perma-Spike pierce increased per spike (70 → 80).
1-0-0 Spike Factory (on right) dispensing a bigger stack
Crosspathing in BTD6 (Top: Path 2, Bottom: Path 3)
In BTD6, the Spike Factory gains a red larger spout instead of turning its feet red.
Following a Version 22.0 nerf for Bigger Stacks in BTD6, the cost of Bigger Stacks has always increased by $100 with each generation of BTD games.
Interestingly, the prices for the Bigger Stacks and Faster Production upgrades were swapped in BTD6 compared to BTD5, costing $700 and $800 respectively instead of $800 and $700 in BTD6, but this only happened during Version 22.0 and its mini-updates.
The upgrade icon for Bigger Stacks in BTD6 actually shows 10 spikes. In BTD5, the upgrade icon for Bigger Stacks only showed 5 spikes. In BTD4 there are also 10 spikes in the upgrade icon, but it's 5 groups of spikes arranged in a cross shape.