The three big American automakers, General Motors, Fiat Chrysler and Ford, have all picked up the pace in their plans to save on labor costs by moving production to Mexico. While luxury cars, SUVs and trucks will still be made in the U.S., all three Detroit-based auto companies are shifting the production of smaller cars to Mexico.

The shift from production in Detroit and elsewhere in the U.S. to Mexico has already been underway, but according to a Reuters report on Monday, GM, Ford and Fiat Chrysler have all accelerated plans to make compact cars that are popular in the U.S. in Mexico.

According to Reuters' unnamed sources "familiar with the companies' plans," three new compact lines -- GM's Chevrolet Cruze hatchback, the successor to the Ford Focus, and an upcoming replacement to the Jeep Compass by Fiat Chrysler -- are all slated to be made in Mexico.

The accelerated exodus of Detroit's production of compact cars to Mexico is a result of the shifting balance of production costs and profit margins, according to the report. The big three are looking to reduce costs in compact and midsized cars, as profit margins have shrunk or gone negative over the past years.

Larger and more expensive vehicles, however, are not part of shift to Mexico. Profit margins for these vehicles tend to be larger in general, and sales of bigger cars, trucks, and SUVs in the U.S. have been strong in the past few years -- largely thanks to the huge dip in gas prices.

With consumer prices for gas dipping around or below two dollars per gallon across the country, fuel economy just isn't as high of a priority. And so buying large, less efficient, vehicles has become more attractive to consumers.

Those large trucks, SUVs, and high-end cars tend to generate profits of $10,000 or more per sale, according to the report, despite being made in the U.S., where autoworkers demand much higher pay than their equivalents in Mexico.

For example, newly negotiated labor agreements with Detroit's big three have produced the first raises for veteran autoworkers in a decade, and reportedly provides a path for workers to earn up to $30 per hour after eight years with the company. Meanwhile, Reuters reported that Mexican labor rates in 2014 averaged about $5.50 per hour.

On top of the vast differences in profit margins and production costs for compact vehicles between the two countries, GM, Ford and Fiat Chrysler are facing stiff competition in the small car market from foreign competitors.

Just last month, Toyota became the biggest car company in the world by sales, according to the Financial Times. Toyota's RAV4, a compact SUV, along with the Corolla and Camry are among the top ten best-selling cars in America in 2015, according to AutoBlog. Compact cars from Honda and Nissan (the latter of which has been increasing production capacity in Mexico) round up the other spots in that top ten, while none of the small cars from GM, Ford, or Fiat Chrysler made the cut. The three top-selling vehicles in America for 2015, however, are all trucks made by Ford, GM, and Fiat Chrysler, respectively.

Moving any production to Mexico is likely to be controversial, especially ahead of the 2016 election, and all three companies responded to Reuters with official statements that either softly denied or obfuscated on the reported plans.

Ford said it was "committed to continuing to improve competitiveness and to invest where it makes the best sense for our business." GM responded to Reuters saying the "vast majority" of small cars for the U.S. market were "produced domestically," while Fiat Chrysler stated it had "made no official announcements regarding the company's future production plans."